Actually, my solution generated considerable further work and the "dog-flea" model keeps cropping up from time to time in unexpected contexts. - Mark Kac

Independence is the central concept of probability theory and few would believe today that understanding what it meant was ever a problem.

I then reached for a time honored tactic used by mathematicians: if you can't solve the real problem, change it into one you can solve.

In the summer of 1930 my academic future, however, was not uppermost in my mind. I had been stricken by an acute attack of a disease which at regular intervals afflicts all mathematicians and, for that matter, all scientists: I became obsessed by a problem.