Showing that a system admits spontaneous symmetry breaking requires introducing a weak external source field that breaks the symmetry and gives rise to a preferred ground state. The system is then taken to the thermodynamic limit after which the external source field is switched off. If the vacuum expectation value of symmetry non-invariant operators is nonzero in this limit then there is spontaneous symmetry breaking. Physically it means that the system never leaves the original ground state into which it was placed by the external field. For global symmetries this occurs because the energy barrier between the various ground states is proportional to the volume, so in the thermodynamic limit this diverges, locking the system into the ground state. Local symmetries get around this construction because the energy barrier between two ground states depends only on local features so transitions to different gauge related ground states can occur locally and does not require the field to change everywhere at the same time as it does for global symmetries.