Why scientists care
The Pauli exclusion principle states that no two identical fermions can occupy the same quantum state. In an atom, no two electrons can share all four quantum numbers.
This rule is fundamental to electron-shell structure and therefore to the architecture of the periodic table.
A concrete example
Two electrons can share an orbital only with opposite spins; no two electrons in one atom share all four quantum numbers.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.