On a January morning in 1995, McArthur Wheeler robbed two Pittsburgh banks without a mask, having rubbed lemon juice on his face. He knew lemon juice was used as invisible ink, and he had tested the theory by photographing himself with a Polaroid, which came out blank. He was arrested that night, hours after the surveillance footage aired on the evening news. "But I wore the juice," he told the police. A Cornell psychologist named David Dunning read about the case and started the research that became the Dunning-Kruger effect.