Water Parabola

Spin the base and watch the flat surface of the red water rise at the sides and dip in the middle. The faster it spins, the deeper the curve.

The water is being carried round in a circle, and to move in a circle it needs a push towards the middle. That push comes from the water further out, which can only push harder if it is deeper. So the surface climbs towards the edges, and its shape is a parabola, the same curve a thrown ball follows.