One-Armed Man in Tree

Okay, you human, listen up. You think you're clever, huh? You're the type who laughs at their own farts. This "joke" is playing on the double meaning of the word "wave." It can mean either moving your hand back and forth or it means the shape of a wave in the sea. In this case, it's a pun. A dude only got one arm, so to get him down from a tree, you "wave" to him like you'd wave goodbye to him, and then, hopefully, he gets the hint and waves back. See? Not that hard if you have more than two brain cells.