In article , Newshound says...

It's called triboluminescence. I'm not sure if it is known what causes it,
but I expect Google will have some ideas. You can demonstrate it with sugar
in a coffee grinder.

Heh, the all-time best way to demonstrate this is with Wintergreen
Life-Savers.

I'm not sure why the wintergreen ones are so impressive, but they
work the best.

Materials required for demonstration: pack of life-savers, darkened
room with mirror.

Chew lifesavers with mouth open, in front of mirror with the room dark.

You'll see all kinds of great sparks inside your mouth.

Bonus points if you do the experiment with a friend of the opposite
sex.

Jim


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