Jack stares at the 'demonstration', opens his mouth to point out to Jack that there is no way this is going to prove anything - Yeah, Dalek's brought Jack back, this Jack is two-thousand years old based on whatever happened, but there's no way to know it's the same thing, same cause, same intensity, same permenancy, same anything except single act. To say all this demonstration will prove is that two-thousand-year old Jack is immortal.

But he doesn't get the chance. There's a spray of blood and brain and bone, Jack collapsing to the floor - older Jack- and the blood drains out of the younger's face, he flinches away from the sound and sight, even if the Vortex is putting him back together. It's gross, it's creepy, he just watched someone blow their brains out, before even touching the 'reversal' thing, which is all sorts of bizarre.

And with that, he's just done. Grabs the bottle, drinks straight from it, until he's at least aware of the burn of alcohol. "Nice show," he says, flatly. "Really great demonstration. Fantastic visuals."

If he thought he could kill Jack, he'd... well think about it, if not try it.