The exercise is interesting in terms of the effects on the brain because it works in about four or five different ways. One of the most obvious ways is blood flow. And so if you get your heart working, your brain's going to be filled with oxygen-rich blood and nutrients. That's the main way that we thought it helps. The other way that's interesting is it's been thought that exercise produces new neurons. So exercise induces the production of growth factors, one of what's called BDF. And it actually stimulates the production of new brain cells. Now when I was in school 20 years ago, we were told you can't get any new brain cells. So when you're born, that's your lot. You know, you don't really make any more. But more recently we found that exercise is a really good way of stimulating brain cell production. And some of these are functional. And so just this notion that something that you can do can generate new brain cells is a really great thing to think about. So we were wondering why exercise helps the brain. And what one theory is it just reduces stress. So maybe it's not that your blood is coming to the brain, maybe you're less stressed and then that was something that imaging allowed us to test. And so we scanned a whole lot of people with high cortisol levels. So if you're stressed, if you're angry about something or even if you're stuck in traffic, cortisol levels can be very high. One of the things we found is that the people with high cortisol levels lost brain tissue fast. Well that's a serious problem. So as soon as you know that's true, you can look at ways of reducing your cortisol. And so that's a very easy thing to do. I mean we can get less stress by exercising, walking, taking breaks. And so imaging established a physical connection between something in your blood and the cortisol that's a sign of stress and actual physical change in the brain, that's very useful to know. So you can take care of your brain and there's a lot of ways we know that you can take care of your brain, you can eat a good diet, you can exercise, you can reduce stress, you can make sure you're well educated. And these things just build up a sort of mental bank account for the future. And so even though it seems like work is hard, having your building a store of brain connections that you'll need for the rest of your life. So these are practical messages that we've learned from imaging a lot of people.