I was seven.

My dad didn't give me a paintbrush so I made one by taping a chunk of styrofoam to a stick so I could paint my wooden airplane. It was oil based paint.

When my war vet father saw the styrofoam dissolving, he grabbed the can away from me, remembered the cigarette in his mouth, then shoved it back and made me put the lid on first.

And that was when I learned how to make ~~nitro glycerine~~ *napalm.

I remember one time when I was a kid and had read something mentioning spark gap transmitters. I of course found a bit of wire (tie wire because that's what came to hand, not anything insulated) and a radio and was playing around with a 9v battery making little sparks by shorting it with the wire and hearing the radio crackle in response. What I then thought was that if the little battery was making a noticeable effect then a bigger battery would obviously be better.

I got one of the drill batteries and shorted that out with my bit of wire to make a better spark and proceeded to discover that resistive heating is a thing and thin tie wire connected even briefly to a high discharge battery will get very hot very quickly. I ended up with a nice blister line across my fingers and a scar for a few years showing the position I'd been holding the wire...

One of my favourites lines from "Ignition" by John Clark.

It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively.

I have a small concrete patio inside my house that is open so it's perfect for the pets (2 cats and 2 dogs) do poop and pee. I went traveling for 2 days and left the pets home and when I came back there was a lot of pee. I was out of cleaner and, since I'm a genius, I used BLEACH to clean the pet piss. Well, we had to evacuate because I just created a chemical weapon inside my house. Almost fainted.

Looks like when, as a child, I read on a bottle of bleach to avoid mixing it with acid. The first thing my dumb ass did was to look for a bottle of vinegar...

Warning: FAFO is not a good way to learn about hydrofluoric acid.

When working in a 100 degree server room on some solar batteries (AC was still being installed), sitting on the floor in your sweaty underwear and pants will give an 52V positive terminal a path to ground, though the contents of your underwear.

Unfortunately it was significantly on the pain side of the pain/pleasure scale of my nether region.

It's not really science-y, but expanding foam is a type of glue and will glue to everything it can. I got expanding foam everywhere including all over my hands for about a week. They were just crusty, not glued to stuff.

body weight acts upon pool cover... i was a kid and almost drowned

Put some lye and aluminum foil in a cup without a handle.

Place a can over the cup with a small hole.

Wait a bit.

Light the hydrogen.

It will also hurt a lot if your finger is on top of the can when you light it because the can will simply dissapear for few seconds.

I don't know about best, but just happened to me: panicked and cleaned off grease from leather shoes with Windex.

I was trying to concentrate hydrochloric acid and had it in my boiling flask on a mantle. It was taking a while and I realized I hadn't added a stir-bar, so I tossed one in.

Then the superheated hydrochloric acid flash boiled and shot out of the flask like 8 feet in the air. Fortunately, I was outside.

I've got two.

A lab freezer's seal broke in the middle of a humid Michigan summer, so everything got encased in frost. In the process of chipping away the frost, the ink on many of the labels rubbed away, so we essentially had a bunch of mystery flasks. One such flask had a septum that was stuck really tight. When I yanked it out, the recoil caused some of the mystery liquid to splash onto my mesh shoe. Within a couple minutes, my foot started stinging. We later identified the contents to be acetyl chloride, so it was probably reacting with my foot sweat to make acetic acid and hydrochloric acid. I took my shoe and sock off and rinsed my foot in the lab sink.

I was putting sodium hydride into an empty round bottom and a good bit of it got stuck to the ground glass in the neck. Genius that I am, I turned a nitrogen line on with low flow thinking I could blow it into the flask. I didn't realize that the nitrogen had to go somewhere and the only place it could go is back out, blowing NaH all over my face. There was very much safety-glasses-unless-there's-an-inspection culture at my old university, but I was never more thankful that I made it a personal rule to wear splash goggles. Would not have liked for the moisture on my eyes to bubble off.

Not as cool as the original story, but I was working with a big carboy of dilute HCL in a CHEM 101 lab. The previous person that used the carboy had managed to spill dilute HCL all over the stopper. I was not aware of this.

The protocol was to grasp the stopper between middle and ring finger, pull it out, then pick the carboy up with both hands and pour into the beaker. That way, the only thing the business end of the stopper ever touches is the inside of the carboy.

I'd just started pouring when I felt the skin between the two fingers start to itch. It was obnoxious, but I had a heavy piece of glassware in my hands trying to measure out a precise amount. So I ignored it until it started to burn. By that point I almost had enough in my beaker so I topped it up. Then I lowered the carboy and replaced the stopper.

Then I ran over to the sink, turned it on full blast, and washed the acid off my hand. I had a red, tender patch there for days. After that, I always wiped the stopper off with a paper towel before I pulled it out.

Not me but years ago the inorganic lab at my uni was tasked with measuring heavy metals in whale fat and did what they normally did back then to disolve test materials. Mix nitric acid and hydrocloric acid in some heavy duty pure quartz reagent tubes, put in sample and microwave.

Well. Turns out mixing triglyceride (fat) with nitric acid and HCl as a catalyst makes nitroglycerine. And what does that do in a confined space and microwaved

It turns expensive heavy duty quartz tubes into expensive quartz dust and a fucked microwave.

It gets quite hot but only makes nitrocellulose (aka guncotton) reliably with unbleached cotton, which you rarely find in clothes.