Well, we actually use a lot more than that. For a complete discussion of the "10% myth", see:

http://weber.u.washington.edu/~chudler/tenper.html or
http://www.exn.net/Main/JaysBrain/jb-19980128.cfm.

We don't use all of our brains all the time, though. If you look at images that show the active areas of the brain, you'll see that some portions are active when others aren't. Different areas are used for speaking, recognizing faces, coordination, etc.. Even though we use most of our brains, we never 'use them up'.

There's a lot we don't know about our brains. We still don't know exactly how learning and memory work, but it looks like it involves our brain cells making new connections. We don't get new cells, they just hook up in new ways. Animals that live mostly by instinct (like earthworms) have most of their brain connections pre-programmed. They can learn, but not much. We are born with very few instincts, but can learn complex new things all the time. Unless our brains are damaged by injury (wear your bike helmet), disease, drugs, or lack of oxygen, we can never use up all of our capacity to learn. In fact, the more we use our brains, the better we get at learning.

If a part of our brain is damaged, we can sometimes recover completely. This is particularly true for children under the age of 10. It seems like our memories are not stored in one place, like a book in the library, but are somehow spread through large parts of our brains.

Is it better to live by instinct and never have to learn anything, or to have to learn nearly everything? What happens if there's no one to teach a "learner"? What if you did everything by instinct and couldn't learn much, then the environment changed? Do other animals learn from their parents? Which ones do and don't? What do they have in common? Do animals that learn more live longer or shorter than average, have more or fewer offspring than average, live in certain types of environments? What do you think the connections are between these lifestyles and the ability to learn?