In order to compose a piece of music, write a short story, paint a still life, brain cells must be summoned to help out. Those of us who have suffered over school assignments know that words don't appear on a blank sheet of paper miraculously, we must rack our brains just to write something coherent. Brain racking is basically brain exercise.
We all know that solving math problems exercises our brains, but creative activities such as composing music, writing poetry, creating computer graphics are also valid brain exercises. This is by no means a new theory, but it raises an interesting question: who is smarter -- a retired nuclear scientist at 60 who does nothing but watching TV all day or an energetic artist at 70 who paints every day?
Somebody could make an argument that watching TV is not all brainless, but that's a different debate altogether.