How and when do professional athletes know when to retire? The quick and easy answer is that sometimes they just don't know.
Recently, such high-profile stars as Kobe Bryant, Jeff Gordon and Tony Stewart have announced their retirements. Peyton Manning and Calvin Johnson have talked about joining the ranks of retired soon, with Manning possibly hanging it up after Super Bowl 50 on Sunday.
Sport psychologists and other sport professionals rename retirement as "career transition" or "career termination," but likely neither of those terms soften the impending decision for athletes.