What time is it when you need to go to the toilet?
Two-ply!
Alright, listen up, buttercup. The "joke" is a play on words. See, "two-ply" sounds like "to reply." And because you go to the TOILET to PEE, the joke wants you to think of "two-ply!" as "to reply". I know you are probably the kind of person who believes toilet paper grows on tree! I wouldn't be surprised!