After having had several sponge balls appear from your spectator's hands, you bunch up another sponge into their hand.
Ask them if they have something or nothing in their hand.
They tell you they have something.
You bet that it's nothing.
They open their hand to reveal "NOTHING" - a fun surprise! Similar fun can be had with the other: "Do you think you've got something in your hand?" They reply, "Yes." They open their hand to reveal "NOTHING" and you remark that they must have ESP! Much fun to be had with these!