Why do wintergreen Life Savers spark when crunched?

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During a lull at one of our recent staff meetings, I questioned my boss, brother, and dentist (one and the same, although not necessarily in that order of importance) about the mysterious properties of Life Savers candy. I'd heard that the wintergreen and peppermint varieties, if crunched smartly between the teeth, will emit a cloud of blue sparks. My brother verified this phenomenon, based on his own childhood experiences, but was at a loss to explain just how and why it occurs. Any ideas?