Deposit the money and you get one, and only one drink.

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Money is the input; soda is the put.

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The pump will only deliver one type of gas when selecting..

When you dial a phone number, you may enter one number at a time.

Functions are used in programming.

Basic structure of functions in programming.

Each letter is going to one person only, but a person may get multiple letters.

Each citizen my only vote for one candidate, but each candidate my receive multiple votes.

Video games depend on inputs and outputs -- a function.

A technical definition of a function is: a relation from a set of inputs to a set of possible outputs where each input is related to exactly one output.