Here’s a fun science experiment to try at home. You’ll need a cotton ball or a Q-tip (generic is fine), a lemon, some water, a bowl, a spoon, and some paper. You’ll also need a light bulb — incandescent, because you’ll need its heat, not its light (and a halogen will get too hot) — but that’s for the second half of the experiment. Squeeze the lemon juice in a bowl and add a little bit of water. Mix it up and then dip the cotton into the watered down juice. Take the wet cotton and write a message on the paper. (It doesn’t really matter what you write, but “Surprise!” gives this a nice touch.) Wait for the paper to dry. The message will disappear.

Now, expose the paper to a small amount of heat, courtesy of that light bulb. (Be careful not to light the paper on fire!) Wait a bit, and your message should appear in brown. You’ve created invisible ink.

Here’s another experiment. (Please do NOT try this one.) First, rub lemon juice on your face. A lot of it, all over. Then, go to a bank, and write them a note — using real ink, as the teller needs to be able to read it even if he or she doesn’t have a light bulb nearby — which demands that they empty their cash drawer and hand the contents over to you, “or else.” Walk away scot free as your face, covered in the invisible ink-slash-lemon juice, makes it impossible for the police to figure out who you are. It’s the perfect crime!

The first experiment works. Lemon juice oxidizes when it is exposed to heat, and in doing so, turns brown.

The second experiment, though, obviously doesn’t work. Or, not so obviously, if you were Pittsburgh-area resident McArthur Wheeler, who tried to pull off that same crime in April of 1995. When he was arrested, he was surprised, apparently telling police “but I wore the juice!” as they showed him his picture on surveillance cameras. (Before the crime, Wheeler had taken a Polaroid of himself, and didn’t appear; police believe that the film was bad or that the acid in his eyes made him unable to correctly line up his shot.) Wheeler, of course, went to jail.

While Wheeler’s idiocy lends itself to an easy punchline, it also created the basis for one of psychology’s more fascinating theories, now called the Dunning-Kruger effect after the pair of researchers who came up with the theory. In 1996, a Cornell professor named David Dunning read about Wheeler’s crime and, to him (and virtually everyone else), incomprehensibly stupid strategy, and realized something. The New York Times summed it up brilliantly: “If Wheeler was too stupid to be a bank robber, perhaps he was also too stupid to know that he was too stupid to be a bank robber — that is, his stupidity protected him from an awareness of his own stupidity.” Working with his research assistant, student Justin Kruger, he co-wrote an article published in the Journal of Personality and Social Psychology titled “Unskilled and Unaware of It: How Difficulties in Recognizing One’s Own Incompetence Lead to Inflated Self-Assessments” (available here). The article, leading with Wheeler’s crime, theorizes that where we know the least, we’re truly vulnerable, because we don’t realize how little we know. We therefore overestimate our abilities and are capable of believing nearly anything.

Just ask McArthur Wheeler.

Bonus fact : The term “scot free” isn’t a slight at people from Scotland. It comes from the Old English phrase “scotfreo,” with “scot” meaning “royal tax” and “freo” meaning “free.”

From the Archives: Miracle Berries: A fruit which makes lemon juice taste sweet and then turns you invisible. Okay, not the second part. But the first part is true!

Related: Invisible ink!