The magician borrows a quarter from someone and, after making sure that it is clean, bites it in half.
Before going out on the street, the magician obtains a quarter. On a string of evenings where there is nothing pressing to do, the magician sits down in front of the television, bites down on the quarter, and grinds away at it with slow, steady jaw motions until it is worn clear through. This results in a split coin with natural-looking teeth marks on its edges. The pieces of the split coin are put somewhere safe while the magician visits an orthodontist, and then hidden in a back pocket when street magic time arrives.
On the street, the magician stops a stranger and asks to borrow a quarter. After looking at the quarter a bit, the magician says, "This looks pretty nasty. Have you been keeping it in your back pocket? I'm not putting that in my mouth. Here, let's use my coin."
The magician drops the borrowed coin into a pants pocket and removes the two halves of the split quarter, being careful to hold the halves together as if they were a whole coin and not two halves. The rigged coin is then placed half way in the mouth and the loose end "bitten" off.
The magician generally will spit the piece into a hand to show everyone, making them run away in disgust or fear. And the magician has just made a quarter!