what you don't use...
"What you don't use you eventually Lose"
Think about it. If you decided to sit still (like a statue) in one position for just one hour, what would happen to you behind and your legs? You start getting those "needles" that prick you. Why, because at that point your legs and butt are so uncomfortable they are screaming for help - relief. They need to be exercised a bit so that the blood can flow through the veins properly. This goes for very muscle you have in your body; they all need to be exercised regualrly, otherwise you end up losing what you don't use. The unused part becomes weak and dysfunctional until eventually, you lose it. That's why people go through physiotheraphy so they can re-train their muscles to function properly again expecially after an accident.
The same case applies to the brain. It is a muscle too that needs constant exercises by providing it with enough mental exercises. If you do not exercise your brain, it starts screaming for help and when you don't listen to the screams, eventually you start losing parts of your brain. That's why some people suddenly start complaining of being forgetful, being unable to quickly do some mental sums, not being able to retain important phone numbers, etc. Why? Because they stopped exercising their brain/mind.
As you go about your day to day activities I'd like to challenge you to evaluate your routine; are you providing any opportunity for your brain/mind to be exercised? Are you allowing yourself to use your brain often? The brain is the only tool each one of us has that can transform our lives.
To quote Michael Jordan (RIP): I believe we are powerful but we don't use our minds to full capacity. Your mind is powerful enugh to help you attain whatever you want.
Like someone else said, "The mind is like a parachute. It works best when it is open".
If you don't want to lose that grey matter between your ears, then start using it. Remember, use it or lose it.