Why do midgets laugh when they run?
Because the grass tickles their balls.
Midgets Laugh While Running
Explanation
Explain Bear
Listen up, you knucklehead. You clearly have a very low IQ. The joke is about short people, specifically midgets, and how they're close enough to the ground that grass can tickle their balls when they run. Get it? It's a physical humor thing, dumbass.
Ball
Stolen joke