A friend of mine nannies one of the cutest kids ever. He told her about his trip to see the Space Shuttle Endeavor. He said, You have to see it! It has a ‘carry on’!"
She didn’t understand, but he couldn’t say it any differently (He’s adorable and three).
So what part of a space shuttle would have a “carry on”?? HELP!
Perhaps he was referring to the Shuttle Carrier Aircraft, the 747 that carried the shuttle on its back? I can see how that could be pretty cool to a kid.
A bit of Googling shows that in its current home, it’s basically just sitting in a hangar, neither carrying nor being carried by anything.
The robot arm?
I’m sure the tour guide explained how it got there. I saw it fly over us twice in Burbank on the back of that jet and it was freaking AWESOME!!!
I doubt he thinks it’s carrion, but I’m sure the shuttle hit plenty of that zooming around. 
The red fuel tank?
TroutMan, I think you’re right. Some spectator had to ask how it got there, kid or grownup. So, it was “carried on” a plane 
How else do parents figure this out? I have no kids, so I have no ideas about daily deciphering from kids. There’s gotta be funny stories about this.
Elder siblings and cousins acting as interpreters (the youngers of any given bunch of kids tend to develop “understandable language” later than the olders, because the younger ones take advantage of the built-in interpreters), and memories of doing so (learning to decipher one toddler’s broken grammar is hard; doing it for toddler number 25 is automatic). Were you older within your family/neighborhood group, if you had one?
Indeed there are. When my son was little, I was opposed to letting him watch television (at least before the point where it became Inevitable) – but we did have a VCR. Because we didn’t have much money, we only had two movies: “Yellow Submarine” and “The Empire Strikes Back.”
So when he persistently called an elevator a “darth vator,” I knew what he meant. 
Can’t nanny show him pictures of the shuttle from various angles, including the carrier aircraft, and ask him to point to the carry-on.
Could he be trying for ‘crayon’? The big external fuel tank looks sort of like one.
This gets my vote. The proper name is “Canadarm” which could be corrupted or misheard as “carry on.”
It would be a prominent feature on the shuttle if displayed deployed.
I would ask the kid to draw a picture of the carry-on. Of course, depending on the kid’s art skills, that might just raise even more questions than it answers. “What’s that thing over there that looks like a six-legged horse?” “That’s you, Mommy!” “Oh, very good, it looks just like me!”
Of course, depending on the kid’s art skills,
He’s three. Not many skills of any kind and certainly not the ability to make objective drawings.
A quick trip to the California Science Center’s website shows that the external tank isn’t part of the exhibition yet.
It also shows that the shuttle is displayed without the Canadarm extended, so this makes my vote less likely.
I have two kids, the youngest is 1. She’s just starting to have any kind of meaningful verbal communication. Sometimes we just can’t figure out what she’s saying. Other times it’s clear. And then there are times when we can figure it out in context, such as when she kept asking for “muggy” for weeks, and finally combined the word with pointing to a bottle of milk."
As kids get older they forget these early struggles with language. My mother insists I used to call a fire engine a “woot” and referred to Batman as “at-bat” but I have no recollection of any of this.
My first thought was that it referred to the cargo bay. That’s where you put the stuff in that you carry on the shuttle.
Maybe the exhibit is run by the TSA and you could only carry-on less than 3 oz of liquid?
I also vote for “Canadarm,” as I’m sure even if it’s not deployed on the actual exhibit, there are pictures or videos of it in operation somewhere in the museum. Maybe even a separate spot with a model or replica just of that piece on display (no idea, never been). The robot arm would be something probably pretty cool to a 3 year old, and the word kinda fits.
I really looked for something here - http://californiasciencecenter.org/exhibits/air-space/space-shuttle-endeavour/space-potty - that would make sense, but couldn’t do it
From what I know of little kids, it’s more likely to be something that sounds like “carry on” than something that actually does any carrying on.
So I’d also think that “Canadarm” is the best bet.