Five mornings a week, my husband goes to the health club, gets on the stair-stepper, sets the timer, and buries his nose in a book. Recently, he noticed an amazingly fit middle-aged woman who seemed to run circles around everyone, took few breaks, and rarely even broke a sweat.

"It's not fair," he complained. "By the time I'm dragging myself off to the showers, she's hopping back onto the stepper for another session."

One day he came home with a sheepish grin. "Well," he said, "I just found out -- they're identical twins."