Size, weight, and magnetic properties. Some or all of these, depending on the mechanism.
As an aside, ever see the old pay phones with the 3 holes on top for nickels, dimes and quarters? Back in the day when phone calls were a dime(!) we used to carry a thin slip of card paper called a "phone fooler" in our wallets. If you slipped the phone fooler down the dime hole/slot to just the right spot, you could put a penny in the nickel slot to make a call.