Within two days last week, two remarkable stories shook the sports world, both of them bittersweet.

Days after winning her 72nd LPGA Tour title, Annika Sorenstam, 37, one of the best female golfers the world has seen, told us this would be her last year on the tour. The next day, Justine Henin, 25, the No.1 women's tennis player in the world, winner of seven Grand Slam singles titles, announced she was done.

Sorenstam said, "The time is right."

Henin said, "There are no regrets. I did everything I had to do in tennis."

How do athletes know when it's time to quit, especially when they're on top? How did baseball's Sandy Koufax bring himself to hang it up at age 30 after winning 27 games that season? How did NFL icon Jim Brown come to quit at age 30 after his MVP season in 1965? How did Brett Favre, seemingly over the hill, know to keep going for yet another season until he brought the Packers to the brink of the Super Bowl?

For most, it has to be a daunting prospect, life in the real world, life without the cheers, life trying to find something to fill the void where there was glamour and excitement and challenge, the feeling that there was still something to be won. One day you're a star, the next you're a guy in the checkout line at the grocery. One night you're turning heads in a Paris restaurant, the next you're at a neighborhood book club discussing Nora Roberts.

Brandy Johnson was a world class gymnast until one day she was suddenly terrified of doing her usual routines on bars and beam. She said, "My brain was telling me it was time to quit."

Life after gymnastics turned out well for her, but she said, "A lot of girls are lost after gymnastics. They've spent their whole life doing something they can't use anymore."

What is it that whispers in an athlete's ear that it's time to go home?

For Sorenstam, a beloved figure with a swing as reliable as daybreak, it was apparently a desire for normalcy, to settle down, to enjoy family life, maybe have a baby. Lorena Ochoa has rocketed to stardom, to No.1 in the world, but it wasn't the presence of the young star that prompted Sorenstam's retirement. The week before she announced that this would be her last season, Sorenstam played three of the four rounds with Ochoa and was 12 shots better in winning the Michelob Ultra Open in Williamsburg, Va.

Henin just wanted out.

Her young life had been filled with tumult. Her mother died when she was 12. She was estranged from her family by an unpopular marriage that resulted in divorce. Her brother was in a serious auto accident and that, along with the divorce, brought the family together again.

"She used tennis as an outlet for her emotions," said her longtime coach, Carlos Rodriguez. "Finally with her life now reconciled, she no longer has the fire that drove her to success."

Sorenstam's departure is a graceful exit. Henin's is an escape. Sooner or later, everyone has a reason to walk away. Knowing when? For some, it's an easy call. For most, it hangs in the air like smoke, real but hard to grasp.