The London moment is the magnetic moment acquired by a rotating superconductor. We propose that the London moment reveals the following fundamental properties of the superconducting state: (i) superconductors (unlike normal metals) know the $sign$ of the charge carriers, (ii) the superconducting charge carriers are $free$ electrons, (iii) electrons are expelled from the interior to the surface in the transition to the superconducting state, (iv) superfluid electrons occupy orbits of radius $2\lambda_L$ ($\lambda_L=$London penetration depth), and (v) a spin current exists in the ground state of superconductors. These properties are consistent with the Meissner effect, however the Meissner effect does not $directly$ reveal the sign of the charge carriers nor the fact that the carrier's mass is the free electron mass nor the fact that a spin current exists in superconductors. Note also that within the BCS theory of superconductivity none of the key properties of superconductors listed above are predicted. Instead, these properties are predicted by the theory of hole superconductivity.