Clever is the opposite of Stupid

A clever person is someone who's Brain is Fit.

Same as an athlete, a person who's physical body is Fit.

Using our brain to remember and to recall memories on a daily basis is part of how we train our brain.

As if we would like to keep our body fit and healthy, we need to exercise regularly and have proper diet. Same goes for the Brain, if you would like to have a fit mind, we need to start using it regularly. Any activities that makes us think, is a good way to exercise our brain.