Some things defy explanation, like quantum physics, why Rihanna took Chris Brown back, and why the hell mookish action star Vin Diesel covered RiRi's ballad 'Stay.'

The video, which features a sad-looking Ri soaking in a tub, looking bluer than the water, plays in the background, while big 'n' burly Vin, known for his husky voice and meatheaded movie roles, is somewhat obscured and to the side. You can't see his face, but that voice? That voice, you cannot mistake.

He can't carry a tune. He can't pull off Mikky Ekko's falsetto, either.

We can't tell if Vin meant this to be funny or serious. If he meant it to be funny then mission accomplished. If he meant it to be serious, well, it was a fail of epic proportions. Vin, stick to the 'Fast & Furious' movies. A dude bro cover of a pop ballad ain't fooling anybody..

When he says "Happy V Day, amore" and gives that wet smooch at the end? Ack!

That turned the tide of this video from comical to creepy. File this one under "Make it stop!"