How do vending machines know your money?
For grown-ups: the science
Vending machines use tiny checkers to look at a coin or bill's special features and see if it is real money.
Coin validators measure features such as diameter, thickness, mass, and electrical properties of the metal. Bill validators use optical sensors, and sometimes other sensors, to examine printed patterns and security features; the machine compares the results with stored rules for accepted currency.
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Transcript
A coin drops into the slot with a clink. How can the machine tell what kind of money it is?
A coin rolls through tiny checkers inside the machine. They feel its size, weight, and what metal it is made from.
A bill slides past a light scanner that looks at its printed marks. The machine checks whether those marks match real money.
If the checks match, the machine adds the money to your total. Then it can let you choose a snack or drink.
So vending machines use little checkers to recognize coins and bills. That is how they know when the right money has arrived.