"The alcohol trouble" joke
During grammar school science experiements into properties of different alcohols:
The residue of each test was tipped down the sinks, which were grouped in threes. There were no U-bends, but each group of sinks emptied into a single box, which overflowed into the mains sewers. Presumably this was intended to retain things like droplets of mercury, which was not banned from use when I was 16.
During the session, my bunsen went out, so I re-lit it with a splint lit from the teacher's bunsen. For safety's sake (!) I dropped the burning splint into the sink, intending to extinguish it with water, instead of waving it around in the alcohol fumes. A small blue flame disappeared down the plughole. Hum, thinks I, I wonder where that's going?
I opened the cupboard 'neath the sink, only to find the drain box, full of alcohol, a roaring mass of flame. Shutting the doors, I called out, "Er, Sir..." just as the inch-thick wooden lids blew off the adjacent un-used sinks. Fortunately, the back-blast extinguished the flames under the cupboard, so the box only sagged slightly!
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