To generate heat they would probably need to eat large amounts of food.

They produce biogas, which can either be burnt to generate heat and power or used as motor fuel; and bio-fertiliser for spreading on fields.

Nuclear reactors generate heat by splitting uranium atoms, releasing neutrons which in turn split more atoms in a chain reaction.

Tyres were used to generate heat while petrol and paraffin residue were found in the fire.

Ginger, cinnamon, cumin, paprika, nutmeg and allspice all help increase metabolism and generate heat.