Aged 15, and "volunteered" to do presentations in the science labs for parent's evening
We were bored out of our brains, as all the parents were in the assembly hall listening to a presentation or some guff like that, and decided to raid the chemical cupboards -well stocked with "harmless" chemicals.

One pile of magnesium powder, Aluminium powder, iron filings and other assorted metal dusts later, we set it alight to watch the sparkly reaction, only to find that we had accidentally set off a thermite reaction, which promptly burned through the heatproof mat and turned a large chunk of the desk underneath to charcoal - along with a large amount of smoke covering the ceiling and a sigh of relief that the fire alarms were always switched off when the labs were occupied.
( MrSanity has maybe kicked the WoW habit forever, Fri 10 Aug 2012, 6:56, 2 replies)

Really? Thermite?
I guess chemistry wasn't your forté?
( Ghoti Fingers, Fri 10 Aug 2012, 7:39, closed)

25 years ago, GCSE chemistry lessons never mentioned it for some reason...
I kinda knew of it through, but specific details were not really in the front of our minds. The length of time ago this happened has also dulled my memory of the exact chemicals involved beyond those I mentioned specifically, but it was most likely a mixture of Aluminium and Magnesium reactions with iron oxide and maybe copper oxide (a rough guess based on hazy memories of the school chemical cupboards)
( MrSanity has maybe kicked the WoW habit forever, Fri 10 Aug 2012, 7:49, closed)