The "13" card trick.. what is the algorithm?

In one of my cles today, the teacher showed us a card trick. you arrange the deck by suit with each suit starting with the ace and ending with the king. then you cut the deck as many times as you want. after that you take the top card and make thirteen piles.. and you go around the piles from 1-13 putting one card on each at a time.. it should work out to get the cards by number sorted together... how does that work.. what is the mathematical algorithm that makes the trick work?