If you’ve ever seen a crime show or a police drama on TV, you’ve probably seen some pretty unbelievable technology. There are computers that can zoom in and "enhance" a tiny section of a video frame. There are machines that can isolate a particular background voice from a muffled recording. Most of this stuff is pure fiction. But one of the most outlandish TV cop tools, a special chemical that reveals invisible blood traces, is absolutely real. The chemical is called luminol.

Let's imagine that while a burglar is trying to rob a house, he cuts himself on some glass from a broken window. Although he is wearing gloves, he still gets blood everywhere. He even walks through it and tracks it around the floor. The burglar doesn’t want to be caught. So the burglar wipes up all the blood. The room looks completely spotless.

The funny thing about blood is that it is almost impossible to get rid of ALL of it. Blood is very sticky. Tiny, invisible traces get left behind, even if you use the strongest cleaner. Police use luminol to make these traces of blood visible.

Here's how it works. Luminol is a powder. Police mix the powder with hydrogen peroxide, hydroxide and other chemicals to make a spray-on liquid. Basically this mixture is like the chemicals in those liquid glow-sticks. This chemical wants to light up, but it needs a catalyst to make the reaction happen. One catalyst that will make it light up is iron. The iron can come from the iron molecules in hemoglobin. Hemoglobin is the chemical that makes blood red - it carries oxygen in blood.

When police suspect that there might be blood somewhere, they spray the liquid where they think blood might have been. In this case, around the broken glass and near the window. If there is any trace of iron, it will make the luminol glow very faintly. In a dark room, it is easy to photograph or videotape the glowing effect and use it as evidence.

Luminol is not perfect. Some chemicals besides blood can act as a catalyst and make it glow. You have to make sure that it really is blood lighting up the luminol. And once you use luminol, it often destroys the evidence. But in the cases where it works, luminol can help a lot. After all the other evidence has been collected, luminol can yield new clues. Police may be able to see footprints, and might be able to match the sole pattern of the footprint with the sole of a shoe. So, no matter how much cleaning the burglar does before leaving the crime scene, his size 12 special-order hiking boots might tie him to the robbery!