TIL of the Isochrone Curve - a shape with unique properties. If you drop two balls along the curve at different points, they will reach the midpoint at exactly the same time.

https://en.wikipedia.org/wiki/Tautochrone_curve

https://www.reddit.com/r/todayilearned/comments/154x1u4/til_of_the_isochrone_curve_a_shape_with_unique/