Someday Marco will undoubtedly learn the truth, whether from Eren or someone else or some weird memory share thing gone wrong, whatever. For now, Eren is just thankful he doesn't push to learn more.

He shakes his head.

"No. Like in the air. Like…in a boat, but it goes through the air."

Literally, an air ship, right? Of course it's an unimaginable thing if you haven't seen it. But flying feels a lot like freedom to Eren.