Today’s xkcd comic:
This comic is about Donald Knuth and his book “The Art of Programming” where he offered to pay money to people who found errors in the logic or code inside the book. In the 1st/2nd panels the black hair stick figure showed the inconsistency and invalidity) of basic logic and thus writes to Dr. Knuth to claim the money for 1,317,408 errors in the book.
The alt text for the comic says:
“Dear Reader: Enclosed is a check for ninety-eight cents. Using your
work, I have proven that this equals the amount you requested.”
So essentially, Dr. Knuth used what the stick figure proved to deduce that he owes her $0.98.