Have you ever noticed how paper turns yellow over time?

Piles and piles of old magazines and newspapers kept in some dusty corner of the house, when taken out during the annual maniac house cleaning, seem way yellower than before. 

Source: Hermes Terulia Literaria

I always wondered what caused that and turns out science has the answer. 

Paper is made of two main components: cellulose and lignin. Both of them serve the purpose of making the wood, from which paper is made, stronger and harder. 

Source: Mauritius.com

Now, while cellulose doesn't change colour that much, lignin turns almost completely yellow when exposed to oxygen in the atmosphere. 

And that is why the paper changes its colour. 

Source: Central Maine

The process of oxidation, especially when combined with the sunlight, completely alters the molecular structure of lignin, thus changing how the compound absorbs and reflects light.

This change causes lignin to turn a yellow-brown color in the human visual spectrum.

Source: Times of India

How to prevent this? Don't keep the paper in humid places because that acts as a trigger for the oxidation process. Also, avoid touching it repeatedly to limit the yellowing. 

Well, now you know.