Another interesting thing about them, they don't actually "swim" like they are commonly depicted. It's not a race upstream to the egg. Sperm are mobile, but their movement is entirely random, they don't know what they are looking for. They just move around and eventually one of them finds the egg. It's actually tiny muscle contractions in the lining of the female reproductive system that move the sperm up to the egg. Kinda like a bunch of blindfolded sheep getting pushed through a field by a shepherd.