On a hot day, there's nothing quite like dropping some change into a vending machine and listening as a cold drink makes its way to the exit chute. It's hard for most of us to imagine a time when we couldn't get a cold drink or a snack — or even rent a movie — from a conveniently-located machine.

There are an incredible variety of vending machines around the world today. In return for payment in the form of coins, bills or credit, vending machines dispense everything from drinks and snacks to newspapers and DVDs. Some of the more interesting vending machines you're likely to find dispense such things as electronics, live fishing bait, French fries, cars and blue jeans.

The first vending machine ever might have been Hero of Alexandria's first-century machine that dispensed holy water when a coin was deposited. Vending machines didn't become popular until the early 1880s when the first coin-operated postcard vending machines debuted in London, England. The first United States vending machine sold gum on New York City train platforms starting in 1888.

Long ago, you often needed to have exact change to use a vending machine. Today, though, vending machines accept coins, paper money and even credit cards. They can also give back change in the form of coins and bills.

How did vending machines get to be so smart? How can a machine tell a quarter from a dime? How does it know the difference between a five-dollar bill and a one-dollar bill?

Vending machines that accept paper money do things that most human beings could not do. They recognize bills based upon multiple observations of different features.

For example, many vending machines that accept bills first identify bills based upon optical scanning. Tiny photocells or miniature digital cameras use images of the inserted bill to “look" for tell-tale patterns that identify each particular bill.

Of course, it's possible to print a picture of a bill that looks just like an actual bill. Luckily for vending machine makers, actual currency has safety features that they can use to verify that real currency has been inserted.

For example, one-dollar bills contain fluorescent ink that glows when an ultraviolet light shines on it. Some vending machines use ultraviolet scanners to measure the glow from a bill to verify it is real.

Real currency bills are also printed using magnetic ink. Many vending machines also use a magnetic reader to detect the magnetic signature of a bill to ensure it's real and determine its denomination.

Still other vending machines have devices that measure bills to make sure they are exactly the right size. Some may also pass a small electric current through a bill to verify it is real. Because of the various safety features in currency, each denomination has a slightly different conductivity.

Coins are identified and verified mainly based upon their physical properties. Although some machines may have advanced technology that allows them to determine the chemical composition of coins, most vending machines simply compare physical characteristics of coins, such as their diameter, thickness and the number of ridges on the edge.

For example, a vending machine can recognize a quarter, because it is 0.955 inches in diameter, 0.069 inches thick and has 119 ridges along its outer edge. This is different from a dime, which is 0.705 inches in diameter, 0.053 inches thick and has 118 ridges.

Did you have any idea vending machines were that smart? Believe it or not, they're actually getting smarter all the time. Many of today's vending machines can be connected to the Internet wirelessly so they can transmit data to their owners. This can help the owners know which items have been sold and need to be restocked.

Some vending machines can also accept orders without being touched. Some touchless vending machines use motion recognition devices to interpret hand signals that users make to indicate what products they want. Still other vending machines allow users to purchase products by sending text messages from their mobile phones!

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