The goggles aren’t a costume.
A beaver has a third eyelid — clear, like a shutter — that slides across each eye underwater. He can see while he works, down in water nobody else can see through. He was born wearing them.
Bix says
“A word you don’t know is just a cloudy bit of water. Luckily I have goggles. They came free with my face.”
What that means here
Here is a child’s murky water: a word they don’t know. The sentence goes cloudy, and they either stop reading or skip it and hope. That is the exact water Bix can see in. They tap the word, choose Ask Bix, and it clears — without leaving the page, and without having to go and find a grown-up.