Oh the irony.

The Washington Nationals are hosting a “Conversion Day” promotion at their new ballpark on Sunday. The concept is not unusual: bring in a piece of gear from any other Major League Baseball team and trade it in for a National hat featuring the curly W logo.

But for this promotion the hat you get in return is made from organically-grown cotton. The sponsor of this "green" giveaway?

ExxonMobil.

In addition, the promotion says there are hats available for the first 10,000 fans, but it also says one hat per fan, “while supplies last.” Hmmm.

The promotion also seems aimed squarely at fans of the Baltimore Orioles, who are the Nats’ opponent that day. After all, the former Montreal Expos are looking to build their fan base in the Baltimore-Washington Metro area. Conversion Day prompted chatter on an Orioles message board.