Tilting at windmills

Tilting at windmills means trying to do something that is impossible or pointless. It comes from an old story about a brave knight named Don Quixote. He thought enemies were attacking him, and he charged at a windmill, swinging his sword. But the windmill had no enemies and the knight did not succeed in defeating it. This idea has been used to describe someone trying to do something that can't be done or makes no sense.