Martin Griffiths, Intuiting the fundamental theorem of arithmetic, Educational Studies in Mathematics, Vol. 82, No. 1 (January 2013), pp. 75-96 ...

The Fundamental Theorem of Arithmetic asserts that any integer can be written uniquely (up to order) as a product of primes. For example, 60 = 2²·3·5 and 42 = 2·3·7.