When I first saw "Airplane!" on cable, I was about twelve years old. There's a scene in that film where Robert Hays, while piloting the crippled airiner, says about one of the other characters, "When he finds out about this... the sh*t is going to hit the fan."...then the scene changes to the control tower on the ground, there's an electric fan on someone's desk and lo and behold a big piece of sh*t goes flying right into it.
...just a dumb sight gag, but to my twelve year old self, it was the funniest thing I'd ever seen in my life up to that point. I was literally rolling around on the floor, holding my sides, with tears of joy running down my face. I had NEVER laughed so hard at anything before and probably never did again.