What do you think?
There is more to thinking than just having an idea in your head.
Thinking is hard work. Sometimes we can feel exhausted after a good thinking workout.
This is because there are different kinds of thinking that use different parts of our brains. Like our bodies, we all have different thinking strengths and weaknesses. Certain thinking skills are easier than others: for example, "remembering" is easy while "analyzing" is more difficult.
Have a look through this book to help you understand more about thinking.