[Korosensei is right not to underestimate his students. Nagisa has seen him pick (often multiple) anti-sensei knives up the same way a dozen times, and it doesn't escape his notice that the process takes his teacher a bit longer than usual. He won't read into it to much… yet. It'll only take another instance or two of him noticing discrepancies for the issue to really get his attention, though.

Nagisa takes the knife and slips it back under his uniform. Now he has to wrap his head around this timeline conundrum.]

That's really confusing. [Still, his eyes light up, breaking his very serious thinking expression for a second.] Hold on, at least… I don't believe that you're lying, Sensei. So if your side of things is true, then what is the chance? Are we going to be safe here for a while, even if there is no cure?