Bottles to feed infants have been in use for many centuries. The first consisted of urns with two openings: one for pouring the liquid into the bottle and the other to be put in the baby's mouth. Predictably, the baby bottle has changed markedly over the last five centuries. The sixteenth-century bottle resembled a duck, with the baby being fed from the beak. Modern baby bottles are cylindrically shaped and are made of plastic or glass. Some modern bottles have disposable plastic liners which eliminate the sterilizing process that sometimes makes glass and plastic bottles inconvenient. A bottle is available that stores formula and water in separate chambers.Unlocking the mechanism mixes the two, allowing convenient transport without refrigeration. Nipples for baby bottles have also changed. Until the end of the eighteenth-century nipples were made from a piece of rolled linen, inserted into the liquid at one end and into the baby's mouth at the other.Sponge leather, wood and dried cow's udder were also used. When rubber appeared on the market during the nineteenth-century, it became the favored material. Strangely, the rubber was used primarily to cushion a metal nipple, sometimes including a spring mechanism to increase the flow. Modern nipples are either silicone or a synthetic rubber-like material with holes of varying sizes to match the child's age and bottle contents.