I bashed my head on an open kitchen cabinet the other day. I saw "stars," those little flashes of light. Where do they come from?
-- Michelle, San Diego
They're an invention of your bewildered brain, Michelle. When real light strikes your retinas, at the back of your eyeballs, it sets any of 100 million nerve endings buzzing. The nerve impulses zap to the vision section of your brain, which translates the pattern into a "picture" of the thing you're looking at. Anything else that sets your eye nerves jangling will have a similar same effect. Bashing your head is the perfect stimulus. But in this case, there's no organized image being transmitted, just random impulses, which your brain can only interpret as specks of generic light.