It’s to do with our DNA! Your genetic make-up is made up from 46 chromosomes, half of which come from your dad and half from your mum. So when a sperm is formed, it’s a random process that says which 23 out of your dad’s 46 chromosomes it will contain. The same goes for the eggs that your mum makes. This means that no two sperm or eggs that your parents produce are exactly alike. In turn, this basically means that the chances are incredibly small that a mum and dad will have two children with exactly the same genetic make-up from two different sperm and eggs. In fact, the chance is something like 1 in a trillion!