With which of the two groups would you place these two hexagons? Although 8 and 9 are both composite numbers, they are somewhat different in character from 6, 10 and 12. They each have only one prime factor. 8 is a power of 2 and 9 is a power of 3 while 6 and 12 have both 2 and 3 as factors and 10 has both 2 and 5 as factors.

What is really going on here? Let's look at these more closely.