Asimov tells a story in his autobiography of having trouble in an English Literature class in college. Being usually a straight-A student, he arranged a meeting with the professor to discuss his difficulty in the class. The professor was less than helpful, stating, "You know what your problem is, Mr. Asimov? You just can't write!" At which point, Asimov pulled out the check he had just received that day for one of his short stories from his pocket and waved it in his face, saying, "I got this for my writing." And walked out.
Awesome, indeed; however, the above combines two separate incidents. When the professor told Isaac he "couldn't write," Asimov's reply was, "I'll thank you to not repeat that slander to my publishers." The incident with the check occurred when a customer in the Asimov family candy store took umbrage at Isaac's failure to say "Thank you" at the close of a transaction. "My son," said the customer, "made fifty dollars this week though hard work. What do you do to earn a living besides standing here?" "I write," answered Asimov, "and I got this for a story today," and he held up the check (for about $64, I think).
Then, there's the time where he managed to outwit one of his friends' professor while he was at university. The professor, at the end of class, had placed mathematicians among people that were prone to believe in fantasy because they believed in i, the imaginary number (square root of minus one, for those that don't know). Asimov said that i could be as real as any other number, and the professor dared him to hand him a piece of chalk equal to i. Asimov countered demanding that the professor hand him half a piece of chalk. The professor broke a piece of chalk in two, but Asimov said that it did not count, because it was one piece of chalk, not half a piece, that he couldn't be sure that it was actually 50% of the original piece, and ended up stating that he could not speak about the reality of i when he couldn't even understand the reality of 1/2. The professor banned him from the class, and Asimov left the classroom while he was laughing.