One of these came up in an exam question and then I found this real live example. Here the heat is generated internally by the bacteria that are causing the kitchen waste to decompose. This is done by aerobic respiration, in which glucose and oxygen are converted into carbon dioxide and water. It is an exothermic reaction so thermal energy is released. The bacteria work best at 37 degrees Celsius. The insulation means they don't transfer too much heat to the surroundings in winter and cool down. The handle means that even bacteria at the bottom can be given a turn with the oxygen. If more bacteria are participating in the aerobic respiration, the compost will be made much sooner.
