Bryce Harper hit his 40th home run of the season during the Nationals' 12-2 win over the Phillies on Wednesday. He's the seventh player to reach the mark in his age-22 season -- joining Alex Rodriguez, Juan Gonzalez, Johnny Bench, Eddie Mathews (twice), Joe DiMaggio and Mel Ott.

That's some good company.

A Phillies fan caught the ball and smartly traded the milestone souvenir for another baseball ... that Bryce Harper signed:

MLB.com's Bill Ladson asked Harper about the moment postgame:

"A fan threw it down. He had a pen, so I signed one of the balls for him. I threw it back up to him and I got the [home run] ball. I'm thankful for that. It's pretty cool to have that kind of ball."

On Phillies fans:

"They are great fans here. They know baseball. I think when somebody does something special like that, these fans appreciate that. … I was very thankful."

It's still early, but we're guessing that piece of memorabilia might be worth a small bit of change someday. Whatever you do, just DO NOT play a game of sandlot baseball with it. You never know what could happen ...