What is special about sperm?
A fertile man makes around 1000 new sperm per second, in the testes. Each carries half the instructions (genes) needed to make a new person, packed into its head. The rest of the cell is a long tail, which propels the sperm towards the egg at about three millimetres per minute. The neck is crammed with mitochondria, which help make the energy needed for the journey. Once they're released, sperm usually survive for 48 hours.