The pun punchline to any number of grade-school level jokes. Following is a retelling of this joke which I learned in the second grade, and which has been dear to my heart ever since. The sophistication of this particular joke is evident in its requiring a "straight man", for the completion of the joke.

Wise Guy: Ok, for every question I ask you, answer with "rubber balls and liquor", ok?
Straight Man: Rubber balls and liquor. <snicker>
WG: Not yet! Ok, so what do you like to eat for breakfast?
SM: Rubber balls and liquor.
WG: What do you like to eat for lunch?
SM: Rubber balls and liquor.
WG: What do you like to eat for dinner?
SM: Rubber balls and liquor.
WG: When you see a cute girl, what do you do?
SM: Rubber ba... Hey wait!!!

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A variant of the joke uses "Rubber buns and liquor" (thanks Ælien), which definitely makes more sense at the punchline, but who ever heard of rubber buns? Seriously...