Two fish swim into a concrete wall. One turns to the other and says "Dam!"
(BTW, remember in The Breakfast Club when Bender is climbing from the closet back to the library via the ceiling crawlspace, and he's telling himself a joke as he does? And he falls through the ceiling into the library as he reaches the punchline? I wondered for years what the punchline to that joke was...and now, it turns out, Judd Nelson was making it up as he went. Hurg!)