Mathematics
Curvature in Riemannian Geometry and General Relativity
Quick fact
In general relativity, the presence of mass and energy literally tells spacetime how to curve, and that curvature then tells objects how to move. This geometric description of gravity perfectly explains phenomena like the bending of light around the Sun.
Why this is interesting
You've likely heard that gravity is the curvature of spacetime, but what does that really mean? How can empty space itself be bent?