I wouldn't say "impossible" but as close to it as you can imagine, in particular if the number n is big, and it must be big if you can't find more or less easily its two prime divisors, EVEN after being said it only has two!

This is the reason why we can have some security when paying, working and having fun with a computer: some codes are based precisely in that (for example, the Public one): you can know the number n, but only the receiver of a message knows its prime divisors.

Just for fun, find the prime divisors of 251646847 (i - this is a product of two primes, ii -don't even dream in trying to find a prime that divides this number by hand, lest you'll get a severe hand-and-brain tiredness)

Tonio