What Do You Do When Your Co-Worker Steals Your Lunch? [POLL]

Imagine working all morning and building up a great appetite only to find that someone else has eaten your lunch.  Irritating!  So what do you do?

The other day I decided that I was going to bring my lunch to work.  It was a very busy day so by the time I was ready to eat, it was already three in the afternoon.  I went to the company fridge and half of my lunch was gone.  It really upset me.

I am not one to let someone go hungry.  All you have to do is ask.  But for you to go behind my back, be sneaky and steal my lunch is crossing the line.  And what is worse is that this isn't the first time that it has happened.  So what should I do?

I've seen decoy bags that look like they have mold on them, but would they really work?