You Asked:

How do instant cold compresses work? — Skip, Vincennes, Ind..

Marshall Brain Answers:

Here’s a demonstration of an instant cold pack:

You can see that the cold pack is at room temperature (21 degrees C, 70 degrees F) at the beginning of the video, and falls to below freezing shortly after activating it.

If you ever open up one of these cold packs, it is very simple inside. There is an inner plastic bag filled with a liquid and a handful of white granules. When you activate the cold pack (either by twisting or hitting the outer bag) the inner bag ruptures so the liquid mixes with the granules.

The liquid is water, and the granules are ammonium nitrate fertilizer. When they mix, an endothermic reaction takes place that absorbs heat from the surroundings. This video is helpful: Exothermic and Endothermic Reactions.

If you want to try creating your own endothermic reactions at home, get out a thermometer. Most any granulated fertilizer will get colder (but not as cold as pure ammonium nitrate) when you mix it with water. The vinegar + baking soda reaction is also endothermic.

More info:
- How Refrigerators Work
- Demolition and Blasting Charges

You Asked:

What would happen if you took alcohol intravenously? — Balaram, Bhubaneswar, India

Marshall Brain Answers:

First let’s state up front that injecting alcohol is a bad idea for reasons we will discuss below. Do not inject alcohol. But it is an interesting thought experiment that helps to understand the effects of alcohol, so let’s jump in.

Most people get their alcohol by drinking it, say in the form of beer or wine or moonshine. Ethanol molecules are absorbed into the bloodstream through the lining of the small intestine in the same way that food molecules are absorbed.

The thing about drinking alcohol this way is that it takes time for the alcohol to get absorbed. The amount of time depends on things like what you are drinking, how quickly you drink it and how much food you have in your stomach. It might take 20 minutes for you to notice significant effects.

Meanwhile the liver is busy processing the alcohol to eliminate it from the bloodstream (you exhale some of the alcohol and urinate some of it, but the liver does 90% of the processing). That takes time too. According to the article, “As a rule of thumb, an average person can eliminate 0.5 oz (15 ml) of alcohol per hour.” So if you drink 1 ounce of 100 proof moonshine, you are drinking 0.5 ounces of alcohol. It is going to take time for that alcohol to enter the bloodstream, and an hour later it will be mostly gone.

This graph shows what happens when a 75 kg (165 pound) person drinks 5 ounces of 100 proof moonshine containing 60 grams [*] of ethanol. You can see that it takes more than an hour for it to all get absorbed and for the BAC (blood alcohol content) to peak. Then it takes 5 or 6 hours for the liver to eliminate all of that alcohol. You can also see that food in the stomach slows down the absorption rate and the height of the peak. This alcohol calculator shows approximately the same effect.

Now let’s say that instead of drinking the alcohol, you inject pure ethanol straight into a vein [**]. For example, say you inject 2.5 ounces of ethanol (the amount of alcohol in 5 ounces of 100 proof moonshine) right into the bloodstream. The big difference is the time. Instead of it taking an hour for the alcohol to absorb, the alcohol is all there instantly. That’s 60 grams of alcohol straight into a bloodstream containing approximately 5 liters (5,000 grams) of blood. There will be a moment or two where the blood alcohol concentration is 60 / 5,000 – a toxic, possibly fatal level – before that alcohol diffuses into the water and fat of the entire body and falls by a factor of 10 to 60 / 50,000 [***].

So to answer your question, there really isn’t any chemical difference between drinking and injecting alcohol. In both cases you have ethanol molecules flowing in the bloodstream. But there is a big time difference. And because of the time difference you would need to be extremely careful in the amount injected. A mistake could be fatal. It would be good to apply the “don’t try this at home” rule.

[*] – 5 ounces (150 ml) of 100 proof moonshine contains 75 ml of ethanol. That ethanol weighs 60 grams because the density ot ethanol is 0.79 g/ml.

[**] – Note that we are talking about injecting pure ethanol, NOT moonshine. As Brian points out in the comments, injecting moonshine or whiskey into the bloodstream would not be smart because of all the impurities.

[***] – If the person weighs 75 kilograms, why is 50,000 grams used here? Because a person is 60% water and some percent fat. Not all 75 kilograms of the body will absorb alcohol.

You Asked:

What should I study to become a pediatrician? — Talisha, Memphis, Tenn.

Marshall Brain Answers:

The article How Becoming a Doctor Works is helpful here. It outlines the steps you would go through to become a doctor. The steps look like this:
1) Get a normal college degree, also known as the pre-med degree
2) Take the MCAT (Medical College Admission Test)
3) Apply and get accepted to a medical school
4) Pass the United States Medical Licensing Examination (USMLE-1)
5) Complete medical school
6) Decide on a medical specialty
7) Complete 3 to 5 years (depends on the specialty) of internship, residency and fellowship
8 ) Obtain a medical license from the state where you will practice medicine

Pediatrics is one of the specialties. It requires 3 years of internship, residency and fellowship, as shown on this page.

The article gives you an appreciation of (and respect for) the amount of training your doctor has gone through to get to where he/she is today.

If you are in high school and thinking about becoming a pediatrician, the article offers this advice on what to study:

While in high school it is important to get a good general education. This is good by itself, but also prepares you for doing well on the S.A.T. Taking a complete science curriculum in high school is key to having a good science foundation for future science courses in college and medical school. Take 4 years of science in high school, including biology, physics and chemistry, plus 4 years of math (including calculus). It is so important to have a good foundation in these sciences so that you can learn the subject matter in college well enough to earn A’s. It seems inconceivable that someone can do well in the killer science courses in college without having been introduced to these in high school. Getting high grades is the most important component in being accepted into a prestigious college.

However, do not be tempted to take easy courses that allow you to get high grades. This will not prepare you for taking courses in college that are much more difficult than those in high school.

More importantly, high school is the time to learn about the real world of medicine (not the one you see on TV). Do something that gets you in touch with the medical field to see if it is right for you now, before putting in all that effort. Learn about other fields that may interest you so that you may compare them to medicine. Also, for the same reason, take elective courses other than sciences to see if these interest you more.

When I was in high school, I volunteered at my local hospital. I enjoyed my experiences there, especially in the Emergency Room. This was part of the impetus in my decision to become a physician. Many years later while in medical school I selected Emergency Medicine as my specialty. I then completed an internship and residency in Emergency Medicine and have been practicing Emergency Medicine ever since. It is surprising how small decisions at an early age can influence the rest of your life.

For more info see: How Becoming a Doctor Works

You Asked:

Can you die from eating too many bananas? — Yeng, Minneapolis, Minn.

Marshall Brain Answers:

Let’s start with the basics. According to the Food and Agriculture Organization of the United Nations (FAO), “Banana and plantain do not contain significant levels of any toxic principles.” So it is not like there are any toxic surprises in bananas (unlike, say, the cassava plant, listed on the same page, which contains cyanide).

Therefore, the place that people might get the idea that bananas could be toxic probably comes from the potassium that bananas contain. Despite the fact that potassium is an essential mineral, potassium can be toxic in large doses. It is so toxic, in fact, that potassium chloride is one of the substances used to kill people by lethal injection. Too much potassium in the bloodstream causes problems with the nervous system and eventually cardiac arrest if the dose is high enough. An overdose of potassium like this is called Hyperkalemia

So the question then becomes, would it be possible to eat enough bananas to cause potassium poisoning? Since the U.S. RDA for potassium is 4,000 mg or so, and since one banana contains about 400 mg, you would have to eat 10 bananas just to hit the RDA. Obviously that is not toxic – that’s the recommendation.

So to get a toxic dose of potassium from bananas, you would have to eat far more than 10 bananas.

If you look up the lethal dose for potassium chloride on a page like this, the oral value is 2,600 mg per kilogram. If you weigh 75 kilograms (165 pounds), you would need to consume 75 * 2,600 = 195,000 mg of potassium to reach fatal levels. That’s 487 bananas worth. Potassium chloride is only about half potassium, so you might need to eat fewer bananas (containing K rather than KCl) than that to achieve toxicity, say by half. But the result is the same – It is hard to imagine that people who are healthy are going to be killing themselves with bananas. It’s hard to imagine someone eating 25 bananas in a day, much less 250.

The one thing that might change the equation would be a situation where the banana-eater’s body is not processing potassium properly. For example, people with kidney failure who are on dialysis are told to avoid bananas because of the potassium. This dialysis page says, “The mineral potassium is found in many foods, especially fruits and vegetables. Potassium affects how steadily your heart beats, so eating foods with too much of it can be very dangerous to your heart. To control potassium levels in your blood, avoid foods like oranges, bananas, tomatoes, potatoes, and dried fruits.” The kidney is the organ that normally regulates potassium in the bloodstream, so once the kidneys fail there can be a problem.

Some medicines (e.g. Diovan for blood pressure) change the way that potassium is processed, and might also lower the number of bananas needed to reach toxicity.

More info:
- Do bananas have seeds, and if they do, where are they?
- How do you grow a banana tree?
- Top 10 Potassium Superfoods: Way Beyond Bananas

You Asked:

How do automatic windshield wipers work? — Norman, Tampa, Fla.

Marshall Brain Answers:

The following video demonstrates how to activate the automatic windshield wipers on a typical car:

As mentioned in the video, the sensor is found behind the rear view mirror. You can see what the sensor looks like in this article:

An infrared beam is reflected off the outside surface of the windshield to the infrared sensor array. When moisture strikes the windshield, the system experiences an interruption to its infrared beam. Advanced analog and digital signal processing determines the intensity of rain or snow. The sensor communicates to the wiper control module to switch on the wiper motor and controls the wipers automatically according to the moisture intensity detected.

In other words, the system shoots a beam of infrared light at the windshield at an angle. The light reflects off the glass-air boundary on the outside surface of the glass and ends up hitting a light sensor. When water gets on the glass, the glass-air boundary changes, and the amount of light that gets reflected also changes. A computer can measure the change and turn on the windshield wipers.

There is a picture of the sensor on this page. See also the light-reflection diagram on this page.

You Asked:

How can a plastic bag enhance the magnetic stripe on the back of a credit card? — Annie, Reno, Nev.

Marshall Brain Answers:

You may have experienced the following: you walk into a store, the cashier tries to scan your credit card, the card doesn’t work, so the cashier puts your card inside a plasic bag. When the plastic-wrapped card is scanned, it works fine.

Let’s start by understanding what is happening when the cashier swipes the card. On the back of the card is a stripe of magnetic tape that is nearly identical to the tape in a cassette tape. The card reader is essentially a tape player that plays the stripe.

The stripe contains a small amount of data – things like the account number and expiration date as described on this page. In order to process your card, the card reader needs to get that data off the stripe without any errors.

The problem is that the stripes receive a fair amount of abuse in your pocket, as well as in the reader. Dirt, scratches and data smearing (where the bits of data on the stripe tend to bleed over into each other) can all make the card hard to read for the scanner.

When the cashier puts the card in a plastic bag, it creates a spacer. Now the card slides through the reader with a slight separation between the data stripe and the stripe-reading head. The separation weakens the signal (the strength of any magnetic field weakens with distance), but it also cleans it up, especially in the case of smearing, because the smeared data is no longer readable by the head due to the spacing.

This page talks about the construction of the stripe:

A magnetic stripe is the black or brown stripe that you see on your credit card, or maybe the back of your airline ticket or transit card. The stripe is made up of tiny magnetic particles in a resin. The particles are either applied directly to the card or made into a stripe on a plastic backing which is applied to the card.

The material used to make the particles defines the Coercivity (see below) of the stripe. Standard low coercivity stripes use iron oxide as the material to make the particles, high coercivity stripes are made from other materials like barium ferrite. These materials are mixed with a resin to form a uniform slurry which is then coated onto a substrate. In the case of a credit card or similar application the slurry is usually coated onto a wide plastic sheet and dried. The coating is very thin and the plastic allows the coating to be handled. It is then sliced into stripe widths and applied to the card during the card manufacturing process. The methods of application include lamination (where the stripe and backing is laminated into the card), hot-stamp (where a heated die is used to transfer the oxide stripe from the backing onto the card after the card is cut to size), and cold-peel (where the oxide stripe is peeled from the backing, and then laminated into the card). Each of the methods have their own advantages and are largely irrelevant to the user of the card.

This page describes the smearing process:

The magnetic stripe consists of magnetised particles embedded in a plastic binder with lubricant. It can be damaged in many ways, including exposure to a large external magnetic field, whether from a permanent magnet or a high-strength alternating current. Damage can also occur by a gradual “smearing” process, in which some magnetic particles are physically dragged from one part of the stripe to another. This transfers a few magnetised particles into zones of the stripe that previously had none, corrupting the data. This is by far the most common mode of failure, and this smearing process is probably under way on your card.

As that article points out, once the smearing has gotten bad enough to corrupt the reading process, it is time to call the credit card company and get the card replaced.

You asked:

How do you build a potato launcher? — Pamela, Kodak, Tenn.

Marshall Brain answered:

The basic idea behind a potato cannon is simple. You have a piece of PVC pipe that acts as the barrel of the cannon. You have a screw-on cap at the end of the pipe that you can open to spray in fuel (usually hairspray). And you have a spark-producer of some kind – often a piezoelectric lighter from a grill or something similar. Put it all together and you have this:

How Spud Guns Work

Here’s a nice explanation of the construction and firing of a potato cannon:

You asked:

I have a 3G connection with a monthly 5 gigabyte bandwidth cap. How much bandwidth am I using when I watch a YouTube video? — Marshall, Raleigh, NC

Marshall Brain answered:

I did some tests with YouTube, and a rough rule of thumb appears to be about 5 megabytes per minute for a “normal quality” YouTube video. For an HQ video it jumps up to 6 megabytes per minute. This means that if you did nothing by watch YouTube videos, you could watch about 800 to 1,000 minutes per month, or roughly 13 to 16 hours of video. Or about 25 minutes a day.

Netflix movies use more bandwidth, as described in this article

We know that the average encoding rate for video streamed to the Xbox 360 is about 2000Kbps. That means one person watching a two hour movie would transfer roughly 1.8GB of data. For high definition movies, the average encoding bitrate is around 3200Kbps and one user would transfer about 3GB of data. Based on the high volume of movies Netflix is doing each month, they are getting a very good rate in the market. I estimate they are paying on average about $0.03 per GB delivered across Limelight and Level 3 and potentially have even a slightly lower rate.

So you could watch 5 to 6 hours of “normal” NetFlix per month.

What about Web pages? I used a tool on the Internet to measure the size of a typical HowStuffWorks web page. According to that tool, the HSW home page is about 350,000 bytes. A typical article page is about 250,000 bytes. The home page and article pages on CNN are 1,150,000 and 800,000 bytes respectively. By contrast, the Google home page is only 15,000 bytes and a result page is about 30,000 bytes.

Let’s assume the “average” web page is somewhere between HSW and CNN and is 500,000 bytes. That means that you could look at about 10,000 web pages per month, or roughly 300 per day. If you are spending all your time on Google or in something lightweight like an email program, it might go up by a factor of 10.

You Asked:

Why does a whip make a cracking sound when someone whips it? — Thomas, Englishtown, N.J.

Marshall Brain Answers:

That cracking sound you are hearing is a sonic boom. The tip of a long whip goes supersonic and makes the sound. The following video demonstrates how to make a whip and crack it:

Here is the tip in slow motion:

This article puts a finer point on it:

Whip Cracking Mystery Explained

“A whip is specially crafted to accelerate a loop in its body to high speeds. The tapering of a whip makes a loop traveling along it speed up by a factor of ten. The lightweight free end allows an extra factor of two to three in speed. For realistic whips, the tip can reach speeds more than 30 times the initial speed,” he said.

See also: Time Warp: Whip Through Soda Can

You Asked:

How do you check the temperature in an oven? — Joy, Lafayette, Ind.

Marshall Brain Answers:

There’s a thermometer for that – an oven thermometer in fact. You can’t use a “normal thermometer” because most of them don’t have a high enough temperature range. An oven thermometer will go to 500 degrees F or more. They come in several styles. An inexpensive oven thermometer is made of metal and glass so it doesn’t melt, like this:

If you want something more advanced you can go digital, like this:

Maverick OT-03 Oven-ChekTM Digital Oven Thermometer

The Maverick OT-03 Oven-ChekTM Digital Oven Thermometer lets you know the exact temperature of your oven throughout the entire cooking process without ever having to open the oven door. Oven temperatures can vary significantly from the set temperature showing on the oven dial. The Oven-Chek OT-03 gives you the average temperature of the oven during the time food has been cooking, after the initial warm-up period. Audible alerts sound and visible alerts flash when the oven is too hot or not hot enough.

See also:
- How Thermometers Work
- How do self-cleaning ovens work?
- Oven Questions
- How to Choose an Oven