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-Water has a very high specific heat capacity (4.184 kJ/K kg), whereas coals (and lava) 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 blood in the foot will carry away the heat and spread it. On the other hand, coal has a poor thermal conductivity, so the hotter body consists only of the parts of the coal which is close to the foot. -When the coal cools down, its temperature sinks below the flash point, so it stops burning, and no new heat is generated. -Firewalkers do not spend very much time on the coals, and they keep moving. -Calluses on the feet may offer an additional level of protection, even if only from pain, however most people do not have calluses that would make any significant difference.