Coins are passed into and out of a bottle, and a coin doubles in the hand of the performer.
Coins passing into and out of a bottle:
The coin is of slightly larger diameter than the neck of the bottle. It looks ordinary, but it has been modified in two ways: first, a very fine groove has been made around the outside diameter of the coin, and a strong elastic band, of the same colour as the coin and fitting the groove, has been placed in the groove. Second, the coin has been cut into three pieces, as shown: the cuts are VERY fine and not obvious unless the coin is examined very closely. The performer takes up the coin in one hand and conceals it in his fingers; s/he places this hand around the neck of the bottle, bends the doctored coin enough to slip it into the neck of the bottle, and with a sharp blow at the butt end of the bottle forces the coin to leave the neck. It can then be displayed to the audience and rattled around to prove that the coin is indeed inside the bottle. Getting it out is accomplished by inconspicuously guiding the coin into the neck of the bottle and giving it a few sharp raps on the butt end: the coin will then slip out through the neck and resume its shape in the performer's hand.
Coins doubling in the performer's hand:
The coin is double to begin with: a thin sheath of metal, of the same colour as the coin in question, is created by placing it over the coin and hammering it so that an image of the coin is created (of one side only, to be sure), and a slight raised rim created. This is placed over the coin and comes apart when shaken in the closed hands.
For the bottle trick, the best effect is achieved if the performer takes the coin up in the hand that holds the butt end of the bottle. A little misdirection allows him/her to shift it to the hand holding the neck of the bottle. Getting it out into the butt-end hand can be done but requires very deft and fast handling: perhaps the performer can drop it and scoop it up with the proper hand.