We were buying spices the other night and noticed a bottle of a strange white powder called alum.
"What is this alum?" I thought.
It turns out that it's a chemical used in pickling cucumbers and fixing dye. It's also used in styptic pencils, and can also be used as a fire retardant, or underarm deodorant (or both, if you're worried about burning your axillae).
Also, it apparently has a sour/bitter, puckering taste that used to be used as a gag in movies and cartoons, such as in Back Alley Op-Roar where "Elmer feeds Sylvester Pussycat alum-laced milk, shrinking his head and driving his voice up several octaves while singing Figaro."
I think the obvious next step would be to get some alum, put it in my mouth, and see if my head shrinks.