Scientists have long known that if you spin a liquid, its surface takes the shape of a paraboloid, a great shape for a mirror. Although well known, the concept was never taken seriously because early attempts to make mirrors by spinning a liquid were less than successful. Furthermore, early researchers only considered using liquid mirrors in astronomical applications. Liquid mirrors cannot be tilted and thus cannot point and track as conventional telescopes do. This limitation was the showstopper in the days when astronomers used photographic plates to record images. However, alternative tracking techniques, made possible by computers and charge-coupled device detectors, have revived interest in liquid mirrors in recent years. Liquid mirrors have two main advantages over conventional glass mirrors: They are considerably cheaper, and it should be possible to build them to much larger diameters. They also have other interesting optical properties: very high surface quality, low scattered light, very low or very high numerical apertures that are easily achievable and variable focus that can be controlled with high precision. The low cost, large size and fast optics are particularly noteworthy advantages. In conventional optics, high costs and manufacturing difficulties often make the design of a large mirror too impractical to be created.