No need to wait for the mercury to drop. In just a few minutes, you can frost a friend's hand.
What to do: Ask a friend to shake your hand and to squeeze it tightly for about three minutes, as shown. As he does, egg him on to squeeze harder. Tell him you feel his hand getting as cold and hard as ice.
When the time's up, have him let go just enough for you to slip out your hand. Tell him again that his fist is ice-cold, then blow on it a few times to reinforce the idea. After a few seconds, tell him his hand has frozen solid.
The payoff: Watch him struggle to unclench his hand, as if trying to unthaw it.