The riddle
What Gets Sharper the More You Use It?
The opposite of a knife
Knives, pencils and scissors all lose their edge the more they are used. The riddle sets up that expectation with the word "sharper" and then gives an answer that behaves the other way. "Sharp" also describes a quick mind, so the word does two jobs at once. Brain and mind are both given as the answer. The brain is the organ, and the mind is what it does when it thinks.
What the research shows
Studies of London taxi drivers are a clear case. To earn a license, a trainee has to learn thousands of London streets. In 2011 researchers Katherine Woollett and Eleanor Maguire scanned the brains of trainees before and after the training. Those who qualified showed growth in the back part of the hippocampus, a region involved in memory. Trainees who did not qualify, and a group of people who never trained, showed no such change.
Sharpening compared
A blade is sharpened by something done to it, such as grinding on a stone, and it gets duller as it cuts. The mind works the other way around: the cutting is the sharpening. Setting the two verbs side by side is the quickest way to explain the riddle to a young listener.
Keeping it simple
The study shows that learning something demanding can change the brain. It does not show that a particular puzzle or game makes a person cleverer in general, and the riddle makes no such claim either. It only says that the mind improves with use.