temperature, density, specific heat capacity, and thermal conductivity. The square root of the product of thermal conductivity, density, and specific heat capacity is called thermal effusivity, and tells how much heat energy the body absorbs or releases in a certain amount of time per unit area when its surface is at a certain temperature. Since the heat taken in by the cooler body must be the same as the heat given by the hotter one, the surface temperature must lie closer to the temperature of the body with the greater thermal effusivity. The bodies in question here are human feet (which mainly consist of water) and burning coals. Due to these properties, David Willey, professor of physics at the University of Pittsburgh at Johnstown, points out that firewalking is explainable in terms of basic physics and is neither supernatural nor paranormal. Willey notes that most fire-walks occur on coals that measure about , but he once recorded someone walking on coals. Additionally, Jearl Walker has postulated that walking over hot coals with wet feet may insulate the feet due to the Leidenfrost effect. Factors that prevent burning Water has a very high specific heat capacity (4.184 J g−1 K−1), whereas embers have a very low one. Therefore, the foot's temperature tends to change less than the coal's. Water also has a high thermal conductivity, and on top of that, the rich blood flow in the foot will carry away the heat and spread it. On the other hand, embers have a poor thermal conductivity, so the hotter body consists only of the parts of the embers which are close to the foot. When the embers cool down, their temperature sinks below the flash point, so they stop burning, and no new heat is generated. Firewalkers do not spend very much time on the embers, and they keep moving. Risks when firewalking People have burned their feet when they remained in the fire for too long enabling the thermal conductivity of the embers to catch up. One is more likely to be burned when running through the embers since running pushes one's feet deeper into the embers, resulting in the top of the feet being burnt. Foreign objects in the embers may result in burns. Metal is especially dangerous since it has a high thermal conductivity. Embers which have not burned long enough can burn feet more quickly. Embers contain water,