Question

If somebody dies, then why do we put Cotton in his/her Nose?

Answer 1

we put cotton in the nostrils of a dead person as we know they won't breathe and it also acts like a barrier against the air as if the air enters the body the body might get swollen. it prevent from germs coming in and going out. 

Answer 2

we put a cotton piece in the nostrils and ears of a dead person 'cos the respiration process stops and furthermore,the air present in the environment enters the body as a result of which the body becomes swollen. 
as we know ,in the dead body,the growth of germs takes place .this is also a reason why we put cotton.simply to prevent germs from coming out.