I was looking for something else, then started wondering what the origin of the phrase “acid test” was…

The phrase “acid test” is connected indirectly to fool’s gold. The term was born in the gold rush years of the mid-1800s. Since gold reacts differently to acid than do other metals, an acid test was done to determine whether the metal a miner found was in fact gold. Now the term refers to any test that produces undeniable results.

That’s interesting in its own right, although I’d gotten a crazy notion that maybe it was related to LSD. Apparently not. I was theorizing along the lines that it had something to do with how they say the CIA was experimenting with LSD as a truth serum, back in the day.