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human
ELI5: Pyrex with boiling water in microwave blew out the door and landed on the floor. Why?

A couple days ago my son is complacently making himself some ramen. He grabs some tupperware, throws in the ramen, leaves out the water, and throws it in the microwave. 3 minutes later, there's smoke everywhere and my wife pulls it out. Took us a couple days to get the smell out of the house. The smell is still in the microwave, and every time we use it it stinks up the house again. So, I've been taking steps to get rid of the smell.

So, last night I'm doing some internet reading, and decide to start with 1/2 cup water and 1tbsp vinegar, and let it steam up the microwave. This is in a 1+ cup Pyrex measuring glass. I run it for 5 minutes, watch it steam up the microwave, it finishes up. I let it sit. I decide to do it again. I look and there's just under a half cup of liquid in the pyrex, so I just start it on another 5 minute cycle. I walk back to the couch and chill. 90 seconds later we are all sitting there, and hear a huge BOOM, and I peripherally see the microwave door BLOW OPEN, the pyrex come FLYING out, and land directly on the wood floor.

We are all sitting there astounded. First words out of my wife's mouth are, "What did you do!?" I examine everything, and there's ZERO damage. There's water and vinegar everywhere in the microwave, splattered all over the place. No damage to the pyrex, no damage to the microwave, no damage to the glass tray. Nothing... I have no idea what I did, and I have never seen that happen before. It was just water and vinegar in a pyrex glass.

I'm confused. What the hell happened?
gpt
You should never boil water in the microwave in a perfectly-smooth container, as you just discovered.

What happens is that the water becomes super-heated.  That is, it rises above its boiling point, but the glass is so smooth that there are no nucleation sites to allow the water to begin transitioning to steam.  So you end up with liquid water that is above its boiling point, and the slightest disturbance will cause it to explosively flash-boil into steam.

Most of the time, you see this with people heating water for coffee/tea in the microwave.  It looks like normal water when they take the mug out, but as soon as they stir the liquid, it "explodes", flinging boiling water everywhere.

In this case, it sounds like the water was eventually disturbed enough, or reached high enough temperatures, to finally start turning into steam.