Thinking about how Grace definitely mistranslated (or at least oversimplified a translation) somewhere with the phrase “you sleep, I watch” because a) Eridians aren't watching anything, they're all blind, and b) Rocky at least can definitely hear Ryland when he's not in the room, but still insists on being physically close to him while he's sleeping.
Grace discovers at some point on or after the journey to Erid that the word he's translated to 'watch' is closer to 'guard' in this context, and that while it's just a social norm nowadays, technically in doing it he's agreeing to fight off any prehistoric Eridian beasts that might attack while Rocky's sleeping.
Beasts that were the same size/bigger than Grace, were adapted for Eridian gravity and didn't need a xenonite suit to breathe.
Rocky has a lot of fun coming up with wilder and wilder explanations as to how Grace could theoretically defend him from these creatures using a combination of ingenuity and Science.
(His suggestions get a lot more elaborate after they both watch Home Alone.)
One night before bed Grace gets curious and asks how Rocky thinks he'd defend him against some Earth predator like a bear.
Rocky pauses.
“Bear is Earth creature with no carapace, internal skeleton like Grace question?”
“Yeah.”
“… If bear attack while you sleep, I hit it once and it instantly die because inefficient internal skeleton breaks into small pieces. Demonstration!”
“Oh. Right..”
