tonight's maths thought:
if you multiply two primes and add 1, then it can't have the two primes as factors, or anything larger than the larger one as a factor either; does this make it always prime? nah because what about numbers between the two? so try TWIN primes, they're only 2 apart! Genius!
(20 seconds of checking later)
(embarrassed look)
(p * (p+2)) + 1 = p²+2p+1 = (p+1)². Obviously, Stuart, you thicky.
My Fields medal retreats into the distance once again.