Just curious--how do you crack a rib and not know it???

Earlier this week, my poor brother in law (referred to as BIL from here on) spent the night in the hospital with pain in his chest area. The ER ran a battery of tests and pictures. Almost 24 hours later, the diagnosis came back from a CT scan. He had a hairline fracture of one of his ribs. Crazy thing is, he has no idea how that happened.

I do know in the weeks leading up to his excitement at the ER, he had been complaining of his side hurting when he and my hubby played golf together. And I am willing to bet my mortgage his wife did not hit him. I know knees, shins, toes, and even thighs are painful victims of table monsters and we generally forget bumping into the monster; leaving us wondering how did I get that bruise on my shin!! But I am pretty sure that a run in with the table monster or even the countertop monster and your rib would not easily leave your short term memory!!

So now I am back to trying to figure out how on earth you can randomly crack a rib! Obviously my BIL is going to be fine. But now that laughing hurts him, he just cannot be his usual comedian of the party come Christmas!!

Any ideas????