Almost every optical instrument, from binoculars to cameras to eyeglasses, contains lenses that were made by opticians. You can discover basic lens-making techniques in this activity.
Opticians make lenses by pouring hot molten glass or plastic into molds. The water magnifying lens does not make as clear an image as a lens made out of glass. (You may have noticed this when you tried to use it to watch TV on a piece of paper.) However, the fuzzy image has more to do with the shape of the lens than with the material from which it's made: the ice lens has a much rounder curve than a glass magnifying lens. In most cases, when a magnifying lens has too rounded a curve, the image will be poor. Very skilled craftspeople also make lenses by grinding glass into curved shapes.