Working at a department store years ago, I sold a light-up makeup mirror to a woman.
It was about a 9" mirror, surrounded with little lights. If it is plugged in, the lights are on, illuminating the mirror.
She brought it back, because she said there were no instructions included.
I explained that it's simple to use. If you plug it in, the lights are on. If you unplug it, the lights go off.
She insisted upon a return.
I was just blown away. I mean, who needs instructions to use a mirror! Or a cord!