Conservation of energy.
Energy used to bend the fork gets converted into heat.
Chemical energy from ingested food yields glucose,
which is converted to ATP,
which is converted to ADP and release energy,
which allows release of ionic calcium in the muscle,
which interact with myofibrils to induce contraction and produce kinetic energy,
which gets converted to kinetic energy applied to the metal thereby bending it,
which gets converted to thermal energy in the metal.
Total efficiency to produce heat from chemical energy through this process... less than 5%?!?