UFC 137 Results: BJ Penn Retires: Looking Back on a UFC Hall of Fame Career

In sports, we learn to cherish every memory. You never know when it may be the last.

Fans piled into the Mandalay Bay Events Center in Las Vegas, Nevada for UFC 137 on Saturday night to see MMA legend BJ Penn face Nick Diaz, but who would've thought it would be the final time we would ever see "The Prodigy" step foot inside a UFC octagon.

"This is probably the last time you're ever going to see me here," Penn told Joe Rogan after losing a close decision to Diaz. "I want to perform at the top level. I've got a daughter and another daughter on the way. I don't want to go home looking like this. I'm done."

If this is indeed the end, Penn will easily go down as one of the greatest fighters of all time. His legend is echoed throughout the sport.

Along with Randy Couture, he is the only fighter to ever hold a UFC title in two separate weight classes. He has competed against and defeated countless legends.

There will never be another B.J. Penn.

Since entering into the sport of MMA, Penn seemed destined for greatness. It took him only three years to earn his Brazilian Jiu-Jitsu black belt, a feat that typically takes many years to accomplish even for the most gifted grapplers.

Shortly after earning his black belt, Penn went on to win the World Jiu-Jitsu Championship, becoming the first ever non-Brazilian to do so in the black belt division.

Most fighters fight their way through a multitude of smaller promotions in hopes of making it to the bright lights of the UFC.

Penn made his professional debut in May 2001 at UFC 31, where he picked up a first round TKO stoppage over Joey Gilbert.

He would go on to defeat Din Thomas and Caol Uno to earn a lightweight title shot against lightweight legend Jens Pulver. "Lil Evil" came out with the majority decision in the epic five-round clash, but Penn's story had yet to be written.

The Hawaiian native rebounded with wins over Matt Serra and Takanori Gomi. At UFC 46, Penn secured the biggest win of his entire career, jumping up to welterweight and making quick work of Matt Hughes in an astonishing upset for the welterweight title.

A contract dispute between Penn and the UFC kept him from the defending the title, and he was quickly stripped of the strap.

It wouldn't be the last time "The Prodigy" tasted UFC gold. He returned to the UFC once again in March 2006. After losing back to back bouts at 170-pounds, Penn dropped back down to lightweight, where he defeated Joe Stevenson at UFC 80 to become the world champion.

It's rare to see such greatness embodied into one individual. People have always looked up to Penn as an icon in the sport. In the minds of fans, he could fly if he wanted to. There was no feat beyond Penn's reach.

Coming into the sport, Penn's goal was to be the lightweight, welterweight, middleweight, light heavyweight and heavyweight champion.

He was never able to accomplish this impossible feat, but in the hearts of MMA fans all over the world, B.J. will always be a legend, a Hall of Famer and one of the greatest professional athletes that ever lived.