André-Marie Ampère was so absorbed in mathematics that, according to an old anecdote, he carried chalk and used any convenient surface as a blackboard. One day in Paris, an idea struck him. He began covering the back of a stationary cab with equations. Deep in calculation, he suddenly watched his “blackboard” move away as the cab departed, carrying his unfinished solution down the street. Source: Walter Gratzer, Eurekas and Euphorias: The Oxford Book of Scientific Anecdotes.