Want to make them laugh?
They think they know what's going to happen- but they get a surprise!
Place a red sponge ball in your hand. Make sure to do it in an obvious manner, so that there is no question that the sponge is in that hand. You may even choose to have the sponge "accidentally" peek out of your hand, as if you don't notice. Ask your spectator which hand the ball is in. He will correctly name which hand (you've made it obvious!). So you tell him, "Well, I guess you've caught me red-handed!". When you open your hand, the sponge ball is gone- and instead, you are holding a Big Red Hand!
Peter Monticup demonstrates the routine using TWO red sponge balls. This item comes with ONE red sponge ball, and you can do the trick just as effectively with the one ball changing to the Red Hand. Of course, you can also use your own red sponge balls with the sponge hand (any 1.5" or 2" red sponge balls will fit).