Paraboloid

The shape made on the surface of a rotating body of cylindrical water is called a paraboloid. This is a little setup in the Science Centre, where you can rotate a cylinder of water by hand to see the effect.

Going off on a little wikiwander from the website linked above led me to the Large Zenith Telescope in British Columbia. This is a telescope with a 6m wide mirror made by rotating a body of mercury in a controlled fashion. Of course, there aren't many such telescopes in the world, because they have one major limitation: they can only point straight up.