Enter the restroom when no one else is around. Turn on the cold water. Let the water run a while so the faucet gets very cold. Keep the water running as you proceed.
Tear the striking pad or strip off of the matchbook. Remove as much of the regular paper from the striking strip as possible.
Fold the strike pad in half lengthwise with the coarse side facing out. Place the folded strip on top of the faucet. Light both ends of the strike pad with your lighter. Light along the sides as well if necessary. Be sure the entire pad burns and turns black.
Move the burnt strike pad by blowing it into the sink into the water. If it has not disintegrated fully, pick it up and throw it away, then thoroughly dry your hands. Turn off the faucet. Wipe the burnt residue off of the faucet with the pad of your thumb.
Return to your friends. Tell them you can make smoke appear from your fingertips. When they are attentive, rub your fingers and thumb together, rubbing the residue until smoke appears.
Wash your hands thoroughly after the trick.