You test them, one at a time, to weed out any that has factors besides ' 1 ' and itself.
When you do that, you find that 2, 3, 5, 7, 11, and 13 are prime numbers.
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Prime numbers less than 30 are 2 3 5 7 11 13 17 19 23 29 Square numbers less than 30 are 1, 4, 9, 16 and 25.
The only prime factor of 16 is 2.
16 is a composite number. All even numbers greater than 2 are composites.
17, 19, and 23. All the other numbers have more than two factors.
All even numbers greater than 2 are composite.