Halfway through the recording, the voice in his left ear switched from English to German. He kept talking and never noticed.

Colin Cherry, a British engineer, ran that test in 1953. He wanted to understand what he called the cocktail party effect, how you follow one voice in a crowded room. He put headphones on each volunteer, played a different recording in each ear, and asked them to repeat the right-ear recording out loud, word for word. The task took all their concentration. Then Cherry tampered with the left ear. He switched it to German, and most volunteers missed the change. He played it backward, and few of them caught that either. The one change most of them noticed was a woman's voice replacing a man's.

You run Cherry's experiment on yourself from waking to sleep. Your brain stops telling you about it. That's how you end up searching the kitchen for the phone in your own hand, or patting the nightstand for the glasses on your nose. You stopped feeling the watch on your wrist years ago, and you'll only notice it the night you take it off.