This frog defies gravity with diamagnetic levitation--many materials that we ordinarily think of as non-magnetic are actually diamagnetic and can be manipulated using magnetism. Learn about several kinds of magnetic levitation at The Real Levitation, and see why frogs were used here.
It's easy to show that grapes are diamagnetic--just see this Exploratorium snack. For an explanation of diamagnetism, see the text at the end of the snack.
You may have seen the Levitron, the spinning and levitating top, in a physics classroom before. Read about how it works here.