Mercury thermometer + school = LOCKDOWN!

[Adeptus_Minor]

Originally Posted by Papa_Complex

Exothermic reaction that liberates hydrogen from water, because it's stripping the oxygen out of it. Makes for nice fireworks. A one gram cube will paint the ceiling.

I had that discussion with my Earth Science teacher freshman year of high school.
We were studying the alkali metals and she was going to show us the ever-popular water reaction. This would have been great, but she got the reactions of potassium and lithium mixed up. She was saying potassium would just skitter along the surface of the water, but the lithium would burn.
I had read ahead in the text and remembered potassium having the more violent reaction.
She insisted she was right so we just let it go. About 10 seconds into the demonstration, the potassium had gone from fizzing to burning and then *POP*... painted ceiling, as you say.
Getting an "I told you so" on a science teacher is sooo gratifying.