180 Days of Ideas for Discussion in Math Class.
(as of 9July2014, we're in overtime!)
A prime number can be written as the product of exactly two "unique" factors. In other words, written as a product of one and itself. One times one would not be two unique factors, therefore it's not prime. Composites, of course, can be written as the product of more than 2 unique factors, zero is next to nothing anyway, and the unit exists all by itself. The fundamental theorem of arithmetic sort of hangs it's hat on this definition of prime numbers.