The phrase "one-to-one correspondence" is sometimes used in mathematics. This describes, for example, how numbers, often learned as abstract sounds in early childhood, can be, one by one, matched with objects. As parents may realize, this is how we teach a child how to count. One-to-one correspondence also shows up in other places in math, as, for example, when rational numbers may be put into one-to one correspondence with the integers, thus showing that there are a countable number of rational numbers.

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