Booby Trapped Chalk
Approx 1990, a Boys Grammar School.
A group of smart, yet stupid, young fellows with brains overheated with all the new learning.

The Good Idea: drill out the centre of a piece of chalk down the long axis, insert a non-saftey match, cover and fill hole with chalk dust.

The theory: Sir uses the chalk and at the first long swooping underline the chalk wears down until the match head makes contact with the board and ignites through friction. Supposedly inert chalk stick spits small flame and small cloud of smoke, laughter ensues.

Practice.
Unfazeable Further Maths teacher picks up infeasibly new stick of pristine chalk suspiciously lying in the centre of the desk and uses it throughout the lesson. Never was a piece of chalk so closely observed as it journey across the blackboard. Every stroke the focus of our acute attention. Eventually a flourish on the blackboard ignites embedded match and Sir pauses, looks at the end of the now smoking match, and quietly announces: "oh. Very good, quite clever" turns chalk around to use virgin end and lesson continues.
Perpetrators look smug at successful jape.

Like all good scientists, we had to repeat the experiment.
The Applied Maths teacher. The nicest chap you could hope to meet. Never a raised voice or cross word. Picks up infeasibly new stick of pristine chalk suspiciously lying in the centre of the desk and uses it for the lesson. Time passes, swooping diagram, chalk stick ignites, Sir looks at sparking end of chalk, burns finger and drops chalk in surprise.
A visibly disappointed Sir turns slowly from the board and calmly declares "I hope none of you boys were responsible for this"
Instantly our smug amusement turns to shame at the hurt inflicted on a decent teacher and we mature in nano seconds. One chap owns up. Sir's look of disappointment is all the punishment we need and nothing more is said.

Bravo, Sirs.
(Rich T has a clever and witty sig, Fri 30 Jan 2009, 0:31, Reply)