thermodynamics - Why does an electric motor burn up when you physically stop it?

As an electric motor spins, the energy from the electricity is 'conducted' to the rotor by the magnetic fields. However, when the motor is stopped, the energy becomes heat and burns up to motor. What causes this heat to be formed? Is it purely generated by the current flowing through the wire or is the magnetic field form the permanent magnets involved somehow? Conversely, why is there less heat when the motor is spinning? I'm hoping this is not just because the motor contains a fan.