This was the first question from my 3 year old this morning. My immediate answer (while I was getting ready for work) was, “Our brains help us do everything we want to do. Without your brain you wouldn’t be able to move, eat, speak, think, and lots more.” I of course then said, "We need to do all we can to protect our brains, so we can continue to move eat, speak, and think. If we injure our brains, we won't be able to do the things we want to do." In other words, I think helmet wearing is super important. J responded, "We wear helmets to protect our brains!"
Thinking about it from a biomedical engineering prospective (older kids/adults), our brains are the most amazing command center controlling our bodies! As we grow and learn, connections from one part of your brain to another are formed. Sometimes connections are easier than others (ie, when you are young, you can learn a foreign language better). Regions of the brains are responsible for different things, so if you injure specific regions of your brain, you might lose different abilities to do things.
On a personal note, I was 25 yrs old and experiencing some pretty persistent, bad migraines that were giving me "mass effects" like blurred vision and numbness. I was diagnosed with a defect called arteriovenus malformation (AVM) in my brain. It's where the veins and the arteries are directly connected, basically a useless vein since the oxygenated blood doesn't go through the capillaries to my tissues, just straight to the vein and back to my heart. Part of the problem is that the arteries are a high pressure system and veins are low pressure. The high to low pressure switch can cause eddies (turbulent flow), which can lead to the formation of aneurysms (where the artery wall balloons out) and bleeds. Talk about a scary diagnosis! Luckily (if there's something to be lucky about), it's a birth defect (so I've lived peacefully with it for a long time), it's gigantic and "diffuse" (5x5x5 cm), and I have inherently low blood pressure.
Here's a picture. You can see the AVM in both views here (look for asymmetry and squiggly lines). The AVM is in my left frontal and parietal lobes of my brain. 5x5x5 cm is a pretty large portion of the brain, and I've been declared neurologically normal, which to me is amazing proof of the adaptability of the human brain (and cardiovascular system too)...
Despite its quirkiness, I love my brain.
**Edited on 1/12/13 to add:
AVMs can be found anywhere in your body. AVMs in the brain are particularly scary since a problem there can lead to a very big problem. I think they are more common than we know. Many people live their lives with AVMs and don't even know they have it. I could have treatments to get rid of the AVM, but the risks of treatment outweigh the benefits since I do not have aneurysms.