Packet milk does not get spoiled for longer time, why?

Pickles have a long shelf life and do not get spoiled for months, why?

What happens when amber is rubbed for longer time?

A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a long time to set as curd?

Why does a person not able to see for some time when he comes out of a well lighted room at night or enters a dark room at day time?

Why does a body stop after rolling down for some time ?

Why does leather get hardened after tanning ?