[He felt like he was really onto something! But then, as he sees the helmet in Fenton's hands, he's left baffled.]

Wait, what?

[It's like his mind is trying to process an impossible equation. He gives an uncertain laugh.]

What - what you talking about, Fenton? Buddy? You, there's no way you're Gizmoduck! You... have his helmet right now, sure! ...But you're just... holding onto it to work on it for him, right? Yeah, that's gotta be it! You shouldn't play sick jokes like this, you know!