An obvious one really. Kids take great pleasure from winding up their teachers because, as everyone knows, children are evil. Sometimes it's accidental such as 'I don't get it' or 'I don't have a pen' but there are definitely occasions where it's deliberate: 'this is really boring', 'you're not as good as our last teacher' or just pretending to be a farmyard animal.

Either way it's easy to get pissed off with pupils from time to time. However, shouting rarely helps. In fact many pupils enjoy seeing their teachers explode, perhaps they even hope that if they're sufficiently annoying this explosion will actually be literal.