Continuing Adventures of My Kid is Fourteen

So she's doing her Latin.

We're on Is Ea Id.

She'd supposed to be filling in the right forms of the demonstrative to go with various nouns.

The Kid: What gender is oppidum?

Me: (as always) Look it up.

Kid: (sighing heavily) (Looks it up) But what if oppidum is gender-fluid?  Huh?  Huh?  Why didn't the Romans ever think of that?