Just looked it up

www.purplemath.com/modules/factzero.htm

If we take the factorial of any number larger than 5, then there will be at least one zero at the end of the number. Why? Because 5! = 1×2×3×4×5; in particular, 5! = (2×5)×(1×3×4), and (2×5) = 10. The factorial of any larger number will have more copies of 2 and 5 (as factors of larger values, like 6 and 15), so there will be even more factors of 10 in these factorials. And every factor of 10 adds a zero to the end of the factorial expansion.