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A prime number is a number that can only be divided by itself or 1 in order to retain a whole number.
For example, 3 is a prime number, because 3 / 1 = 3 and 3 / 3 is 1 but 3 / 2 is 1.5 (1.5 is not a whole number)
On the other side 4 is not a prime number, because 4 / 2 = 2, so it actually has a factor besides 1 and 4.