If you've ever raced the wind across a lake in a kayak or done a perfect cartwheel on the lawn, you know how good it feels to exercise. Your body was made to move. It feels cramped and sluggish , when forced to be still. Your blood pressure rises, your appetite goes askew, your bones become brittle, your muscles ache, you feel tired and become moody, depressed, or ill-tempered. If you had lived in prehistoric times-or even in pioneer days-you would have exercised as you found food, built your home, made your clothes, and gathered your fuel. In between, just for fun, you might have run races, squaredanced, staged throwing contests, slid down hills, and otherwise entertained yourself with games of physical prowess. In the modern world, you no longer have to do physical labor to feed and clothe yourself, but you still need exercise to be fully alive.