The brain weighs just 3 pounds , but it controls everything you do from thinking , learning , feeling even to breathing and your heart beating . Even though the brain is just 2 % of your body weight , it uses around 20 % of all your energy i n your body . Tha t is a lot for such a small organ( 器官 ). There are about100 billion tiny , t iny little cells( 细胞 ) i n your brain. There are so many that it would take you over 3 , 000 years to count them all!