What percentage of my brain is being used?

I would say all of it in one way or another, although there will be some areas which will be more active than others. There's an old urban myth that you only use 10% of your brain. It's absolute rubbish as the brain has such a high metabolic rate. In fact, the part of the brain that has the highest metabolic rate and burns off more energy than any other part is in fact your retina. It's part of the brain itself because it's an extension of the central nervous system. If you look at someone who's had a stroke and damaged a bit of their brain, even though they might be doing a task that doesn't directly seem to involve that part of the brain, the person still doesn't seem to be completely right. They still obviously have some difficulties. So we know that all of the brain is used probably all the time but some bits are recruited and do slightly more some of the time. And the reason we know that is because you can inject substances into people which are used as a way to map out which bits of the brain are more active than others. So you give people a radioactively labelled glucose molecule for example, because the brain loves glucose. Wherever the brain is doing more work, it needs more energy, so it burns more glucose. So in a special scanner, you can see where the radioactivity is being concentrated. That's called a PET scan, and it tells you which bits of the brain are doing what jobs. Because you can get people doing repetitive tasks, if you ask them to just move their right hand for example, and the part of the brain that's concerned with just moving the right hand will light up.