I picked up some "conventional wisdom" when I was a teenager, which said that we replace the cells in our bodies every seven years - out with the old, in with the new; grow new muscle and bone and whole organs.

Except the brain.

For whatever reason, our gray matter was our gray matter, and we wouldn't get any more. A few seconds thought would've told me that this was wrong - if nothing else, children's brains get bigger as they grow to adults and that mass has to come from somewhere - but I carried the idea that the brain is static into my thirties.

Until yesterday, as a matter of fact, when I read an article about exercise and anxiety. Apparently we do grow new brain cells, and exercise makes it happen. If my bookwormy teenaged self had known that, I probably would've gone out for football.

I'm exercising regularly now, though, and feeling extra smart, but I can't say if it's the new brain cells or my new knowledge that's got me that way. In either case, the real news (to people less reliant on conventional wisdom than I) is the way those new brain cells respond to stress.

As a Quit Coach, I'm used to talking to people who smoke to deal with their stress, and one of the most common substitutions we hit on together is exercise. Heart-rate and breathing up, then down, and the quick flush that comes with it; stimulus and response. Or, if nothing else, you're too tired when you're done to think about the things that have you down.

Because the truth is that exercise is stressful. We put pressure on our body, but here the body responds by getting stronger. And it turns out that's what happens to the brain, too; your new neurons are better, faster, stronger than the old ones - and better equipped to handle stress firing across them. I'm thinking of exercise as a steam-roller, flattening out the road so cars can drive through with a minimum of tire blow outs and the traffic jams that would result.

And that beats stimulus and response, stress-so-exercise. Because we can't control the amount of stress we have in life, and the occasions when we have the most stress are often the occasions when we have the least time or desire to exercise; exercising in response to every bit of stress would be absurd. But a bit of regular exercise before the stress gets to its worst can lay the foundations for dealing with it better.

Put that way, exercise is a very direct about-face from smoking. Cigarettes might make you feel peachy in the short term, but tear you down over time; exercise tires you out today, but builds you a better brain by tomorrow.