1.

　　Your brain is the organ inside your head that controls your body's activities and enables you to think and to feel things such as heat and pain.

　　2.

　　Your brain is your mind and the way that you think.

　　3.

　　If someone has brains or a good brain, they have the ability to learn and understand things quickly, to solve problems, and to make good decisions.

　　4.

　　If someone is the brains behind an idea or an organization, he or she had that idea or makes the important decisions about how that organization is managed.

　　5.

　　To brain someone means to hit them forcefully on the head.

　　6.

　　To beat someone's brains out or bash their brains in means to hit their head very hard, so that they are badly injured or killed.

　　7.

　　To blow someone's brains out means to shoot them in the head, killing them.

　　8.

　　If someone has something on the brain, they keep thinking about it.

　　9.

　　If you pick someone's brains, you ask them to help you with a problem because they know more about the subject than you.

　　10. to rack your brains -> see rack