Why do ordinary amines rapidly switch between mirror-image shapes?

Ordinary amines possess a molecular shape resembling a flattened triangular pyramid, with the nitrogen atom at the apex and an unshared electron pair localized above it.1

These molecules rapidly switch between mirror-image configurations—known as enantiomers—through a process of extremely rapid inversion. This mechanism is comparable to an umbrella turning inside out in the wind: the substituents attached to the nitrogen atom move in one direction ("up"), while the nitrogen atom itself moves in the opposite direction ("down"). Because of this constant equilibration, chiral amines cannot be isolated into stable mirror-image forms under ordinary conditions.1