Cleanliness means that there is no dirt, no dust, no stains, no bad smells. The goals of cleanliness are healthy, beauty, absence of offensive odor and to avoid the spreading of dirt and contaminants to oneself and others. In the case of glass objects such as windows or windshields, the purpose can also be transparency. Washing is one way of achieving cleanliness, usually with water and often some kind of soap or detergent. Since the germ theory ofdisease, it has also come to mean an absence of germs and other waste. In industry, certain processes need conditions of exceptional cleanliness which are achieved by working in cleanrooms. A very new idea is that ‘dirt’ may play a useful role in our immune systems.