Water, or more specifically groundwater, often contains microorganisms that carry disease. However, bringing the water to a full boil kills the organisms. The process of boiling items in water serves the same purpose. For example, parents are encouraged to boil baby bottles and cooks are encouraged to boil canning jars prior to first use. The average time of boiling to sterilize baby bottles is 5 minutes and for canning jars is 10 minutes. There are some substances that do not withstand boiling and other methods of sterilization, such as with chemicals, are used in those cases.
In medical situations, sterilization of surfaces and tools is of utmost importance to patient health. While hospitals and other facilities use autoclaves for sterilization, boiling is an acceptable method and it is used for items that cannot stand up to the pressure and steam levels inside the machine. Boiling is often used to sterilize certain metal tools, cloth, rubber and plastic. Items used inside the human body should be boiled for 20 minutes for safety.
Science experiments, such as those used to study the growth of microbes, also require a sterile environment. Petri dishes or other media in which substances will grow are often sterilized in boiling water for anywhere from 5 to 15 minutes.Learn more about Cooking