Find a smoker in a pub and wait until he goes to the loo, usually the person will leave their pack of smokes on the bar. While the person is at the loo, empty his pack of smokes.

When he/she returns, tell him/her you want to show them a cool trick.

Take the victims pack of smokes (the one you emptied earlier) and place it upright on the bar counter. Explain to the victim that his pack of smokes is a Warship.

Take a second pack of smokes with at least three smokes left in the pack, and place it flat on the bar counter, about 15" away from the "War Ship". Explain to the victim that this is a submarine with three torpedoes, and a mission to sink the war ship.

Now ask the victim to guess the distance that the torpedoes must be shot at, to sink the war ship.

First shot (doesn't matter about the distance feet, mile or whatever), take one torpedo (smoke from submarine box) and make as if it shoots out, but it falls drastically short.

Second attempt, the victim will increase his distance, but make sure this one also falls far short of the war ship.

On the third attempt the victim will give a larger distance than the first two attempts, as s0on as the victim guessed the distance, smash his pack of smokes as hard as you can with the palm of your hand, and say "Congratulations, you are on target!" and enjoy the astonished look on your victims face.

Tips:
Some smokers might find this trick offensive
Make the story interesting, e.g. an American submarine is on a mission to destroy a Soviet Battle ship etc.