You can train your mind, which resides in your brain, a unique muscle, but not so much training with physical calisthenics, actually.
No doubt you've heard that if you don't "use it, you lose it" - most often referring to physical muscles. This also applies to our minds to the degree that If we do not USE them, we shall lose our mental or brain capacities. In your effort to keep fit, you need to exerice your mind. (Not your brain directly; it is just the organ your mind resides in). Do you know how to train it?
It is vital to train your mind to concentrate. Before you do that though, you should calm and unclutter it. Three ways to do that are. . . .
(1) a walk or phyiscal exercise that brings in oxygen will help to calm your brain muscle,
(2) to unclutter it, turn aside from the little stuff that has been crowding in and nagging you,
(3) get away to a quiet, peaceful place and give yourself permission to relax, ready to focus on just one thing.
To be truly useful, you must train your mind to have skills of concentration. You will not always need to be super intensely focused. Your imagination, for instance, is much more playful, but if you are to manage your mind and make it most productive, you will need to concentrate.
So practice concentration by focusing carefully on some small matter, such as a jigsaw puzzle, or some other puzzle or problem to solve. A Rubic's Cube would do fine too.
One good exercise for training your mind to read silently and try to grasp the key thought of each sentence - even if we are not sounding out every word and syllable in our head. This will lead to reading faster and faster, but without losing comprehension. Our mind will fill in the small article words - when we let it.
Another way is to try hard to learn something new, such as a language, mathemetical exercises, or to memorize passages of Scripture. Don't worry, we have plenty of room in our mind to store all we want to stash there. We have more brain cells than we can ever use - even if you train your mind to the ultimate.
I once heard a man say in a seminar, that as a child he had found elementary school very difficult. But in high school he was persuaded to take up Bible memory work, and by the time he was ready for college he was a National Honors Student in the USA. The memorization had improved his mind to the point where it affected all his other learning!
Just trying to tell God in prayer how great a Creator He is, how mighty and powerful and that His ways - naming the ones we can think of - are past fully understanding - that effort alone can do wonders for clearing and to train our mind. It opens it up to hear from God Himself, and we are able to receive and perceive great thoughts and ideas that would not come to us otherwise.
Learn to research. Find more memory and word or math games if you enjoy them. Memorize your shopping lists, long poems, monologues, and whole chapters of the Bible. When you are young they may seem to be mere silly exercises. When you are older you will discover some have turned into marketable skills. At very least, they will make you a more capable and effective person at whatever career or work you take up.
Ideally, I think God intended us to use our minds in noble ways that reflect His glory. We should not be selfish and use our minds only for our own amusements.
Always learning and searching for more knowledge is good, but not an end to itself. "The chief end of man is love God and enjoy Him forever," so says the catechism of many Christian churches. That is a great place to start!
Remember that God's mind is so creative and great that He thought up the whole universe, every celestial body, all those gallaxies of stars, and every detail of our earth. We will not match Him! But by waiting on Him for directions we can learn and do things that no one on earth done so far.
Let's take just one example; our creative imagination. It is just one function or aspect of our mind, yet probably the most admired and the most fulfilling.
Some of us get an extra portion of imagination at birth. Unfortunately, many grow up like Topsy, tumbling along like a kitten from one small imaginative adventure to another - with no plan or purpose. It pleases God to see us aspire to higher goals, forge a path that no one has ever gone, and acomplish wonderful deeds never seen before. Such ideals inspire me to train my mind.
Don't just slouch in front of the TV and allow nonsense to be spoon-fed to you. The imaginative and creative partitions of your mind will become numb sponges.
Create for yourself an agenda or development plan for your mind. For example;
1. Concentrate on learning new things and remembering information to make it stronger, sharper.
2. Practice thinking both logically and creatively. Solving puzzles and mysteries would help both.
3. Try to understand deep and wonderful concepts. You'll find a lot in the Bible. Work towards being able to express them clearly so that others can grasp and appreciate them too. Do this orally, but also in a written format.
4. Work out a list of reliable authorities and information sources that you can turn to whenever you lack details or understanding. (God and His Word should be at the top of the list).
5. Devise good filing systems for yourself. You need a mental one to put away things in your memory. You should also have a physical one for your things, your books, and written documents.
6. Start creative projects, even if they will take quite a while to finish. These can be making things, researching something in depth, and working out a clear presentation for others. Some projects will call for more from your mind and imagination than you thought you had, but that is because using your mind always developes it, making it stronger and wiser.
(Just think: if I don't train my mind and use it, Alzheimers maybe the rat that will sneak in!)
7. Another more advanced effort, is to grasp the pertinent information and nuances for making good decisions. The better you can make wise decisions, the more likely you are to find yourself in a leadership role as you continue through life.
8. if you are a Christian you can always resort - in an instant - to praying and asking God. I can vouch for the fact that in the next little while you will suddenly think of a solution or where you will find the answer. That is God answering!
9. If you are stuck on an ethical question; (i.e. which is the right thing to do), it helps if you have read and memorized much of God's Word, the Bible. The more you know about God's character and how He has dealt with others in the past, the easlier it is to pray, then just THINK - of all the things you know about Him and His ways, what is most likely to be His attitude and answer to your deliema? In a short time you will just - KNOW! Or be able to figure it out from the things He brings to your trained mind.