In Shakespeare's play Macbeth, Lady Macbeth is consumed with guilt over a murder. Her mind plays tricks on her: She imagines that her hands are covered in blood and tries without success to wipe it off, muttering to herself, "Out damned spot, out I say!"

A simulation of luminol at work: Before spraying luminol, there's no sign of blood. After spraying luminol, the latent blood traces emit a blue glow.

If Lady Macbeth were around today, she might not seem so deranged. It's easy enough to clean up the visible signs of blood, but it's nearly impossible to remove all recognizable blood traces. Crime scene investigators can find hidden blood even if the criminal has cleaned a room from top to bottom - even if several years have passed!

The magic tool is a chemical called luminol. When luminol comes in contact with hemoglobin (a protein found in blood), it triggers a chemical reaction. This reaction generates a greenish-blue light, illuminating even the smallest bit of blood. The investigators turn down the lights and reexamine the crime scene as it once was.