PDA Alert: Kate Middleton and Prince William!

Prince William and Kate Middleton have done their best to keep their relationship quiet.

But that's a challenge when you're British royalty and all.

Therefore, it was major news across the Atlantic over the weekend when witnesses spotted rare moment of public affection between the two as they walked from a post-polo match party near Windsor, England.

"William spent about 40 minutes at the party after his match, and then picked up Kate and they wandered back to the car," a source told People. "He put his left arm around her back and there was plenty of smiling between them."

There was plenty of bodily fluids exchanged between them later, too, we're sure... if you know what we mean!