Despite being made of snow, an igloo can safely contain a small fire or lamp due to basic physics and material science. When heat from the fire slightly melts the inner snow surface, the water quickly refreezes into a thin layer of ice, forming a hard, airtight shell. Snow is an excellent insulator because it traps air pockets, dramatically slowing heat transfer. Once the ice layer forms, it reflects heat inward rather than letting it escape. As a result, temperatures inside an igloo can rise fr