The magician is given a signed coin. After a great show of parapsychological effort, the magician returns the coin. It has been bent nearly in two.
The magician has three items hidden in one hand, and takes pains to make sure that they stay hidden while the relatively unencumbered hand takes the signed coin. The hidden items are three rocks -- two big ones and one small one.
When the magician has the signed coin in hand, the two big rocks are placed on either side of the coin with a bit of a gap between them. The third rock is placed on top of the coin, positioned so that it is just over the gap. This setup must be made under cover of the hand and not exposed to spectators.
During a moment of misdirection, the magician places the coin-and-rock setup just behind the tire of his or her car and backs over it. This forces the small rock into the gap, bending the coin. The coin can then be produced with a flourish. A little water splashed on the face to simulate the sweat of effort is also a nice touch.