Triboluminescence

“The tree that never had to fight / For sun and sky and air and light / But stood out in the open plain / And always got its share of rain / Never became a forest king / But lived and died a scrubby thing.”
 – Douglas Malloch, “Good Timber”

“Smooth seas do not make skillful sailors.” A proverb – If we calm all the seas, where do they learn to sail?

Friction is where discernment resides. It is where critical thinking begins.