"A non-smoking area in a restaurant is like a non-peeing area in the pool"
It's interesting to point out that in both cases it's diffusion that invalidates the concept.
Though speaking about peeing in the swimming pools, my favorite one is "It's okay to pee in the pool, but not from the jumping stand". This can be used to illustrate the difference between "blatant" and "flagrant" types of chutzpah.
[sent by TY, the second one by GS]