A bat, flying through the night sky, is thirsty. As it flies, it sends out high-pitched squeaks and listens for the returning echoes. It hears a telltale pattern. It hears no echoes form up ahead and the only ones that reflect back at it are coming from straight below. That only happens when the bat flies over a flat, smooth surface like the top of a lake or pond. The bat dives, opens its mouth to take a sip of refreshing water… and gets a mouthful of metal.

In nature, bodies of water are the only large, smooth surfaces around. Waves of sound that hit the surface of still water would generally bounce away, except for those aimed straight downwards.