Yet another anecdote on walking and mind-wandering, this time from Chemistry World:

In late 1922, Wolfgang Pauli took an aimless stroll through Copenhagen’s beautiful streets, deep in thought. Presently, the Austrian physicist met a colleague who remarked in a friendly manner, ‘You look very unhappy.’ Pauli answered fiercely: ‘How can one look happy when he is thinking about the anomalous Zeeman effect?’ How indeed?

Pauli recalled the episode shortly after winning the Nobel prize in physics in 1945 for his formulation of the exclusion principle: each electron in an atom must occupy a unique quantum state. Although he didn’t know it as he wandered around Copenhagen, struggling to explain how magnetic fields split atomic spectra, Pauli was unwittingly laying the foundations for his greatest insight.