Crush a Wintergreen Mint in the Dark and It Sparks

Sugar is a crystal, and crystals do not enjoy being snapped in half. Bite down hard on a wintergreen-flavoured sweet in a completely dark room, and tiny flashes of blue light can flicker in your mouth. It looks like a magic trick, and it is exactly the kind of thing that turns up in illustrated science books, usually with a delighted child holding up a sparking mint. But it is real, and it has a properly serious name: triboluminescence.

The chemist Linda Sweeting of Towson University defines triboluminescence, from the Greek for to rub, as light given off by rubbing, mechanical action and fracture. The word combines the Greek tribein, meaning to rub, with the Latin lumen, meaning light. The effect comes from electric charges. When sugar crystals break, electrical fields pull positive and negative charges apart, and when those charges come back together, the discharge ionises the surrounding air and produces light.

On its own, much of that light is wasted on human eyes. As the Fluorescent Mineral Society explains, breaking sugar produces ultraviolet light, which we cannot see. This is where wintergreen comes in. EarthDate, a programme of the University of Texas at Austin, explains that wintergreen oil, a compound called methyl salicylate, is fluorescent: it takes in the ultraviolet light and gives it out again as blue-green light, which is much easier to see in the dark. The flavouring, in other words, works as a tiny built-in lamp.

That is why wintergreen sweets are the classic choice. The Fluorescent Mineral Society notes that peppermint versions of the same well-known ring-shaped sweets also produce light, but they do not fluoresce nearly as much. To see it for yourself, you need a properly dark room and a few minutes for your eyes to adjust to the darkness. A mirror helps, or a friend who is willing to stare into your mouth while you crunch.

None of this is new. According to EarthDate, the effect was first noted around 1620 by Francis Bacon, who saw sugar crystals give off light as they were broken. So sugar has been putting on this small light show for at least four centuries, most of the time with nobody watching at all. For most of history, it seems, people simply ate their sweets with the lights on.

Sugar is not even the strangest example. EarthDate adds that two quartz pebbles rubbed together in the dark flash with light, and that peeling ordinary sticky tape produces a faint blue line; researchers at UCLA found that in a vacuum, peeling tape gives off X-rays instead, enough to see the bones in a finger. The explanation for the sparking sweet comes from putting two well-understood effects together: charges separating as crystals break, and fluorescence turning invisible light into visible light. The next time you crunch a mint in the dark, you are repeating a four-hundred-year-old observation, with your own teeth as the equipment.