MRIs show that when you think, various parts of your brain light up, depending on what kind of thinking you are doing. “Lighting up” simply means that an area of the brain is active, using energy. (Some very readable basics on intelligence and brain fuction in here.)
Can’t find the reference at this moment but some while ago I read an article about how a kid’s brain behaves when he plays a video game. When he’s given a new game, initially his whole brain (or much of it) lights up, as he tries to “figure the game out”. As he keeps playing, though, the area alight diminishes until there’s just one tiny spark flickering. The gameplay has become a repetitive activity and lost its ability to stimulate much of the mind.
This is pretty much common sense. But it’s a nice reminder that presenting your brain with not only new information but also with new problems and puzzles to solve is likely a healthy exercise. So if you’ve never tried a Sudoku puzzle, give it a shot. Pick a subject that’s never interested you and commit to a bit of reading and study. Even if you’re 50, try to figure out that last row on Rubik’s Cube. Or if you aren’t a chessplayer, take a month to learn the rules and some basic strategy and tactics.
(But after the month is up, for heaven’s sake, stop!)