Di Wu

Light usually behaves like a pedestrian on a two-way street: the path from a streetlamp to a camera matches the reverse path. But near a rotating black hole with broken spatial symmetry, this street becomes one-way. Swap the light source and observer, and the hole's shadow changes shape: from an oval resembling a rugby ball, it turns into a teardrop.

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.