"The Non-Fat Yogurt" Season 5, Episode 7

Everyone is loving the non-fat yogurt at the new yogurt shop. It tastes too good to be true, which turns out to be the case when Jerry and Elaine have the tasty stuff analyzed after Kramer points out their recent weight gains. Jerry attempts to be a better role model and undo the harm he caused when saying F*ck in front of a friend's son.