Isn't the point of a thermos to put "perishable and fermenting foods" in it, like coffee or tea? Like this could totally have been me, like make a thermos-full of coffee one day, then forget about it only to come back to in some weeks just to open it and have one of my eyes blown off

You ferment coffee? Tea I can understand if you're making kombucha, but coffee?

good luck of anything fermenting after dousing with boiling water..