Water Spinner

When a thin, flat, vertical tank of water spins around on a turntable, the water surface forms a parabola. The visitor turns a crank which makes the turntable rotate, and thus controls the speed of rotation. When the tank rotates rapidly the surface forms a deeply curved parabola; when it rotates slowly it forms a shallow parabola. The visitor can also use a magnet to position several rods, which float vertically when the water is at rest. When the tank rotates however, these rods align themselves perpendicular to the surface—they are no longer vertical.
The surface of the water is perpendicular to the net force in the rotating frame of reference.