This video shows you how Nitinol wire (also known as memory wire) works:

You get it warm and the wire returns to a preset shape. How does it do that?

The wire is made from an alloy of nickel and titanium. The alloy’s bizarre abilities were discovered in 1962 at the Naval Ordnance Lab (which is where the NOL in the name comes from).

The reason why the wire behaves this way is because, unlike normal metals, nickel titanium has two different crystal structures. One expresses itself at low temperatures, the other at high temperatures. When it is cool, the nitinol is fairly soft and flexible. As it heats up, it feels more like a spring. It is much stiffer. It remembers the high temperature shape, and when you heat it it springs back to that shape.

To get the wire to memorize a shape, you put it on some kind of fixture (usually ceramic or metal) and heat it to 500 degrees C in an oven for several minutes. Then you quench it in ice water.

This same wire is used in certain robotic applications as “muscle wire”. Heated up, it pulls itself into shape. Allowed to cool it relaxes. Here is one application: