Stealing milk again, eh? How many times will people have to re-learn the same old lesson: if you really annoy someone else, they'll find a way to have a spot of revenge.

In the case of a lab technician friend of mine, he dealt with a milk thief by the simple expedient of adding some phenolphthalein to a fresh delivery of milk immediately after delivery, then retiring to bed to give the thief time to pinch it.

As predicted the thief took that milk. Thereafter no more milk went missing; the thief obviously having weighed up the pros and cons of milk theft. On the pro side, you get free milk. On the con side, you occasionally get to spend a day on the toilet with severe diarhoea, and you don't know when the latter will occur...

There's an old student trick to stop your flatmates nicking your milk: put a drop of green food colouring in it.