Caustic Soda

"Right, this, boys and girls (I went to an all boys' school, but Dr. Marsden tended to ironically use the patter of a  stage magician whenever he showed us something cool or funny) is caustic soda".
At this stage of our lives, none of us has a clue what "caustic" means, but we know soda is an American word for pop, and we've also come across bicarbonate of soda - if not at home then in a previous lesson with Dr. Marsden. So naturally we assume it's something along those lines.
"I want you to form a line at the front and I'm going to drop a little bit onto your thumb". Sounds easy enough, and anything which involves queuing up at the teacher's desk is clearly not going to involve any writing, or anything that could be described as "work" for a while, so we go about it with an enthusiasm rarely seen outside the chemistry labs. We dutifully queue up and Dr. Marsden proceeds to put a drop on the forefinger of the first few boys, who then amble back to their desks, rubbing thumb and forefinger together. After a few have been done, the Doc says:
"Can you describe what it feels like, boys?"
"Sir, it's a bit slimy"
"Yeah, and a bit sticky"
"feels like soap"
"OK, so you'd describe it as 'viscous'?"
"Yeah that's it", a few boys chime in sheepishly, having been taught the word only last lesson, and told to remember it.
"The interesting thing about caustic soda is that it actually has a consistency very similar to that of water"
"Sir, so why is it slimy then?"
"Viscous, boy. It's viscous because it's not actually the caustic soda that's causing the viscosity, it's the cells of skin it's dissolved from your finger tips"
The soft, barely perceptible whisper from the rubbing of 10 pairs of thumbs and forefingers comes to an audible halt and blood drains from about half the faces in the room. A couple of boys strangle a gasp, determined not to look like wimps, but at the same time shitting bricks about how much more the caustic soda was going to dissolve before he got round to telling us the antidote. The other half of the faces instead turned to a kind of macabre excitement - wanting to feel the viscosity of dissolving skin but already knowing the outcome, and more importantly they guessed - the resolution, before getting the drop of flesh-eating liquid.
"It's actually neutralised by water, you just need to wash it off"
I have never before or since, seen a group of 12 and 13 year old boys more eager to wash their hands. In seconds the four sinks in the chemistry lab were surrounded by boys trying to get to the water first, while the rest of the class clamoured round the teacher's desk to get flesh-eating liquid dripped on their skin, somehow more exciting by the fact that we knew what it was going to do, but exciting in a safe way, because you know how to make it stop.
I imagine modern health & safety laws, and the culture of law suits we have now, preclude this kind of demonstration these days, but for all it's barbarism, it was very effective - to this day I think of that lesson whenever I hear the word caustic, and I've never forgotten the properties of caustic soda.