Why do pages of books turn yellow after years of use? --- Arvind, Bhilai, India
Paper turns yellowish or brownish for a couple of different reasons. For example, dust and oil from fingertips will discolor paper over time.
But a big part of yellowing in paper is a chemical called lignin. The amount of yellowing and the time it takes for paper to turn yellow is determined by the amount of lignin in it. When paper pulp is created from a tree, the tree contains cellulose and lignin. The highest quality paper has the lowest lignin content.
So, newsprint is low quality paper. It typically contains 20% lignin or so. If you leave newsprint out in the sun for a day or two, you can see its color changing. The lignin reacts with light and turns yellow.
Typical white copier paper, on the other hand, has less than 1% lignin. It fares much better in sunlight, but given enough exposure it too will turn yellow.
For more info see: Why do newspapers turn yellow over time?