Thoughts claim our attention continuously, and waste our time and energy on unimportant and useless matters. They actually rule our life. We have become so used to this slavery, that we take it for granted, and have become unconscious of this habit, except on certain occasions. While breathing, we do not need to pay attention to each inhalation and exhalation. We become conscious of the process of breathing only when we have some difficulty with breathing, such as when our nose is blocked, due to a cold, or when we are in an unventilated  room. It is the same with thinking. We become conscious of the constant onslaught of our thoughts, and of our inability to calm them down, only when we need to concentrate, solve a problem or study. We are also aware of them when we have worries or fears. Look at the following familiar situation. You need to study something for an exam. You sit comfortably on the sofa with the book in your hands and start reading. After a while you feel hungry and go to the kitchen to eat something. You return to read, and then hear your people talking outside. You listen to them for several moments and then bring your attention back to the book. After a while you feel restless and switch on the radio to listen to some music. You continue to read for a little while, and then remember something that happened yesterday, and you start thinking about it. When you look at your watch, you are amazed to find out that one complete hour has passed and you have hardly read anything. And at this time, you feel you're thinking. This is what happens when one lacks concentration. Imagine what you could have accomplished if you could control your attention and focus your mind! When can you feel you are thinking?
While breathing. While not reading. When you need to concentrate. When you waste your time.