A joke about contrast

I remember chatting with a girl in high school:

I asked her how often you take a bath in winter.

She said twice a week, but she can wash PP.

I panicked. Wash PP?

Why don't you take a bath?

The girl said it was cold in winter, and sometimes she was too lazy to take a bath.

I said, why don't you take a shit and clean it up?

People gave me a white look and said nothing more.

I still don't understand.

Don't understand!