I was reading some simplistic explanations of Pauli's Exclusion Principle (PEP) to explain a group of non-science people, and I came across this: For Fermions, even as pressure builds, no two can be located in the same energy state. This causes them to "stack up" in effect. Only under great force can this(PEP) be overcome. I have a problem with the last sentence. They go on to talk about Neutron Degeneracy Pressure and formation of black holes, but I'm unsure whether PEP is actually being violated there. Can anyone shed a light?