Answer: Triboluminescence

Wint-O-Green candies, a minty offering from the Wrigley division of Mars, will often spark and glow when rapidly crushed. But by what mechanism does a tasty candy turn into a sparkler?

You can thank the phenomenon of triboluminescence for the fireworks show in your mouth. Triboluminescence is an optical phenomenon by which light is generated when material is ripped, crushed, rubbed, or pulled apart through the breaking of chemical bonds in the source material. The most common place people see triboluminescence, thanks to the popularity of this trick among school children, is the open-mouth chomping of Life Saver’s Wint-O-Green candies.

When you crush sugar crystals, the shattering of the crystalline structure frees up electrons. These electrons then collide with nitrogen molecules in the air around them. In turn, the nitrogen molecules begin to vibrate as a result of the extra energy. Most of the time, the release of that energy is in the ultraviolet spectrum and invisible to the human eye—you could smash hard, sugary candy with a hammer all day long and likely never see so much as a visible spark. This is where the particular properties of the Wint-O-Green candy comes into play and turns the invisible, visible.

The principle flavoring agent in Wint-O-Green candy is methyl salicylate (wintergreen oil). Methyl salicylate is fluorescent and, when the ultra-violet rays comes shooting out of the crushed sugar crystals, it absorbs the rays and emits them as visible blue light. When you smash a bunch of winter green candies in your mouth and blue sparks seem to fly about, what you’re actually seeing is the ultra-violet light from the fracturing of the sugar crystals lighting up the wintergreen oil.

Other examples of triboluminescence include diamonds that will glow blue or red when rubbed rapidly or sawn under the right conditions (during the shaping and cutting process), quartz crystals that will create bursts of white light when rattled or rubbed against each other, and certain brands of tape that will emit a soft glow if quickly unwrapped or snapped off the roll of tape.