Question

Sometimes a crackling sound is heard while taking off a sweater during winters. Explain

Answer 1

because sweaters rub with our body and produce charge 

Answer 2

Principle: STATIC ELECTRICITY
When the sweater is worn for a long time, it gets charged because of friction with the under shirt.
electrons of  the sweater are loosely bound with the sweater and hence gets transferred to the shirt giving the shirt a negative charge.
when you take off the sweater, it comes in contact with the shirt and at the place of contact, current(conventional) flows from the shirt to the sweater producing the crackling sound.