Why do pages of old books turn yellow as time goes by?

Everything deteriorates with lapse of time. Thus the old pages deteriorate in condition and turn yellow.

when times go on everything changes, in the same way some chemical reactions occur in paper and it turns to yellow as when times go on food items taste get change.

Paper is made from wood, which is largely white cellulose. Wood also has a dark substance called lignin, which ends up in the paper. The exposure of lignin to air and sunlight is what turns paper yellow. Lignin eventually turns paper yellow because of oxidation i.e. the lignin molecules, when exposed to oxygen begin to change and become less stable.

I accept the points stated by sarala and i think it has a valid point and as a result of paper being prepared from wood and even the wood also turns from green to brown i mean the stem or bark of a plant.

Everything under sun has got wear and tear .So as the pages are made from paper with time with the effect of dust it turns yellow.

But some of the books doesn't turn into yellow, as we feel that those paper are qualitative.Can any one tell why this happens.

I think it might be made up a good fibre or from a high quality paper or from a good. To be frank i dont know and wait what the rest say about that

Thanks for the answer sarala, but lets wait for ravishankar to answer that...

Some papers doesn't turn into yellow, can anyone tell me the reason please.

I too have noticed it in my Father's old books. :)

Even me, can you find out the reason why it happens.some told that is the quality of the paper it depends.