The human brain never stops thinking, sometimes the thoughts might be useful and a lot of times it is the useless stuff that we are thinking about. But that is how our mind is, in a continuous process of thinking. Your thoughts also defines how far your mind can reach or how deep you can think. A lot of people have come with so many theories that eventually lead them to interpret it in practical life. They help us in the invention of things. It is our thoughts that lead us to come up with numerous plans and ideas that can be implemented in real life. A lot of fictional movies, and books that have only hypothetical concepts or characters are just a result of our thoughts.
The thoughts keep coming and going and some even stay. Often the thoughts that we wish to forget, linger on our minds and sometimes it happens so that some thoughts that we so desperately wish to remember, don't come back even after trying hard. Certain thoughts come to us even when we don't want them. They even depend on what time of day it is, mostly at day time you think of work or any other stuff that you want to do and during the night, when all is silent you usually find yourself thinking about your personal life, the emotions at this time are very extreme, so either you think of the good times and it brings you immense pleasure or you end up in a cage of all the bad things that have ever happened to you.
It is surprising how it works, sometimes your mind overflows with thoughts and at other times you're completely blank with no thought at all. You can think of anything possible or not. You can think of things that don't even exist. You can think of the most evil act a person can do and you can think of the most beautiful things. Your mind has both good and the bad ones, they keep flowing like an ocean with no end to them. But you are the master to them, you have the power to decide which ones you're going to stick by, you decide which ones rule your heart. They decide the purity of your heart. They'll also affect your actions and your behaviour. Your thoughts decide what kind of a person you're gonna be. If you let the bad ones take over your heart, you end up as an evil person but if you choose the ones that are good, it makes your soul beautiful. So try to attach your heart to the beautiful thoughts that come to your mind. There will also be bad ones coming to your mind side by side, don't let them stay in your mind. Fill your mind with all the good thoughts so that there's no room left for the bad ones.
It is the ability of humans that makes them superior to all beings on earth. So make use of it to come up with new inventions, to make this world a beautiful place and not to destroy what you already have.