Robert B. Mann

A black hole's shadow is a dark spot against the bright light bent by strong gravity. Usually, this spot is an almost perfect circle with a smooth edge. But if a black hole is not quite what Einstein's theory predicts, a sharp kink, like a corner on a crumpled piece of paper, can appear at the shadow's edge.

A pot of boiling water is covered with two lids — an inner and an outer one. The thermometer under each shows its own temperature, but their harmonic mean (a special averaging method where you add the reciprocals) always remains unchanged. Heat seems to obey an invisible rule.