what is the difference between its and it's ? give a logical reasoning behind it ?

Its: to denote possession.

A chameleon can change its colour.

It's: short form of 'It is'/ It has

It's been raining for a week. ( Here, It's is used as a short form for 'it has')

It's a great movie. ( Here, It's is used as a short form for 'It is'.