If someone hits you, why you see stars

Matt: I bashed my head on an open kitchen cabinet the other day. I saw stars, little flashes of light. Where did the light come from? — Michelle, San Diego

They were made up by your bewildered brain. When real light strikes the back of your eyeballs, it starts any of 100 million nerves buzzing. The nerve impulse zaps to the vision sector of your brain and it does what it’s trained to do, tells you there’s light out there. Anything else that sets your eye nerves buzzing will have the same effect. Bashing your head is a perfect stimulus. It shakes your nerves and it rattles your brain, which then figures it’s just random light and creates “stars.”