[He's not necessarily calling Marty a liar, okay, he's just saying this guy is probably insane or stoned out of his mind or probably both. Grounders, he understands. Alternate universes, sure, he understands. Science, space ships, interstellar travel, that all makes sense. Giant old gods bursting forth from the ground is a little much.

His lips part a little as Marty defends himself, that muted skepticism falling into bold-faced undeniable (possibly judgemental) disbelief.]

I don't suppose whatever you were smoking happened to be laced with anything, right? Experimental drugs, maybe jet fuel? That stuff will fry your brain- not to mention set you on fire.

[Call it a public service announcement. Helpful advice for everyday life.]