The skin of a snake is a physically protective layer. It helps prevent injury, prevents drying out, and helps snakes to minimize friction. Because snakes lack limbs, their bodies are in contact with some surface at all times, which produces a huge amount of friction. As a result, they have to both minimize friction in order to move forward, and generate their own friction in order to generate enough propulsion to move. Scale and skin orientation helps to accomplish this, and it has been demonstrated that nanostructures on their scales may play a role in this process. Some snakes polish their scales. They secrete an oily substance from their nasal passage, and then rub the secretion all over the scales. This is done at varying intervals depending on the species of snake, sometimes frequently, other times only after shedding or molting. It is thought that scale polishing is used as a method of waterproofing, and it may also play a role in chemical messaging or friction reduction.