Jack: Look, I know I should know this by now. I swear it'll be the last time I ask. These wormholes we go through, they're not always there, right?
Sam: No, sir. It can only form between two open gates.
Jack: What's with the worm part? The worm thing. I...I don't get that.
Sam: That's just a metaphor.
Jack: Right. I knew that.