Sun tan lotion that tans itself

I have sunglasses that darken in the sun. They’re photochromic. So, like my sunglasses, this lotion has a photochromic dye mixed in. As you lie in the sun, whatever lotion that's directly in the sun gets dark, but the lotion that isn't, doesn't. See?

So, it gives you a self-leveling tan. And some neat effects, too.

Put your hand on your stomach for a minute, and get a cool afterimage.

Lie in the shadow of a picket fence, and you'll get stripes. As with the sunglasses, the afterimages are good for about ten minutes of walking around before they fade.