Skin is the outer covering of the body. In humans, it is the largest organ made up of multiple layers which guards the underlying muscles, bones, ligaments and internal organs. Because it interfaces with the environment, skin plays a key role in protecting the body against pathogens - biological agents that cause disease or illness - and excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D and B. In the human body, the skin provides the evaporation surface, while the energy comes from the body's core heat.