Season 7, Episode 8: "The Pool Guy"

Because this world is your sanctuary...

-...and if it comes in contact with this-- -Yes, it blows up.

If you know, what did you tell Elaine for?

I didn't know. Kramer told me about the worlds.

You couldn't figure out the worlds theory for yourself?

It's just common sense.

Anybody knows you gotta keep your worlds apart.

Yeah, I guess I slipped up.