Prime numbers only have two factors {1 and itself}

eg

2 has {1,2}

3 has {1, 3}

5 has {1, 5}

Some numbers have an odd number of factors:

1 only has one factor {1}4 has {1, 2, 4}9 has three factors {1, 3, 9}

Can you find any other numbers with an odd number of factors?

What do these numbers have in common?