Lateral language

We were watching a David Attenborough documentary together:

David Attenborough - Even more so for the female monkey when she is carrying a baby.
Ella - Female is a girl.
Me - That's right.
Ella - And the letter is the boy.
Me - Letter?
Ella - Yes, the letter is the boy.
Me - What letter are you talking about?
Ella - No, the letter. Like the one the postman brings.
Me - [Straining my brain] Aaaaah! Mail! The boy is called 'male', sweetie.

She did a similar thing the next day when we were at a swimming spot at the lake marked with buoys. Ella had already told me that she knew that they were called buoys. Then she asked:

Ella - Can I go and swim near the boys and girls?
Me - Sure, you can go swim near those kids. Just don't get too close to their body boards.
Ella - [pointing at the row of buoys) No, at the boys and girls over there!

Then I remembered her saying - in reaction to me referring to the buoys as "those round floating things": "and the square ones are the girls."

Isn't language fun?!