It's the mixture of the bone shooting off, the shattering of the window, the yell of someone who wasn't mortally wounded—and the look on Magne's face seals the deal on César losing his shit, suddenly unable to find the ability to breathe.

"Bone! Less mass!" César tries to say more, and just manages to choke out: "Than people—!"

And then the room becomes silent again as César is seized by a fit of laughter so strong that it takes his breath away. César pumps one fist into the air triumphantly while he desperately clings to the back of a chair with the other to remain standing. He doubles over, wheezing.