In "Harry Potter and the Goblet of Fire," the fourth book in J.K. Rowling's blockbuster series, Harry eats a plant called gillyweed that allows him to grow fins and gills so he can swim underwater for an hour.

Wouldn't it be great if you could change your body so that you could swim better or run faster when you worked out?

Well, now Canadian researchers say you can do just that by tapping into mental and physical powers you never knew you had.

In a study, scientists had trained cyclists do a strenuous workout in a hot room (95 degrees) with 50 percent humidity. Then, one group got nine training sessions in motivational self-talk.

They were taught to think things like, "I'm doing well" or "I can handle this" and to make sure they didn't think, "My legs are killing me" or "I'm sweating so much." The second study group trained as usual.

After two weeks, everyone went through the tough, hot workout again. This time, the participants who had practiced motivational self-talk handled the discomforts better and were able to continue cycling for 25 percent longer than they did earlier.

The lesson: Your mind really can win out over matter, but you gotta believe.

So the next time you're walking or jogging and feel like you're just not going to make it through the next mile, allow your brain to think, "I can do this!" –

Chances are good your body will follow right along.