A colleague (who is unreasonably organised) has organised our office secret santa today. Now nothing unusual about that but due to lots of cheating (ie it becoming known who is buying a present for whom) he has devised a system such that
1. No one (including him) knows who is buying a present for who
2. There is no chance of anyone being given themselves to buy a present for.
How has he done it?
(a clue - he has used sealed envelopes and labelled the envelopes)