When I was a student, I used to get together with my roommate every day at lunchtime. He was a graduate student in experimental psychology, I was an astronomy undergraduate. Yes, that’s important.

One day, I bought the coffee, two identical styrofoam cups of black joe, with 1 packet of sugar, 1 of creamer, and a little wooden swizzle stick each to stir them with. We both drank our coffee the same way.

I prepared one cup, stirred it, and got ready to fix the other when my friend stopped me.

“Wait, don’t stir mine with that stick. There are oils in the wood that give the coffee a bad taste. Let me get a plastic spoon to stir mine with.”

“Nonsense”, I replied. If there is some impurity in the wood, its in such a low concentration that no human being could possibly detect it.” I didn’t know this for a fact, of course, but I was feeling in an argumentative mood.

He vigorously disagreed with me, so I proposed an experiment to settle the question.

“I’ve stirred one with a stick, I’ll stir the other with the plastic. You look the other way and I’ll shuffle them so you can’t tell which is which. Taste both, and you tell me which was stirred with the stick.”

He agreed, and we performed the experiment. Sure enough, he couldn’t tell them apart. He didn’t even attempt to guess. I thought I had proved my point.

“No”, he protested, “this is not a valid experiment. The two cups are not identical, there may be small but significant differences in the amount of cream and sugar in those packages, there is no experimental control to eliminate the possibility of variations in the two cups other than in how they were stirred.”

Was he right? Or was he blowing smoke? See the following post, “Spoiler Alert”, for my logic.