Aqua

"They're not," Aqua said with a smile like a ripple. "Aqua is my name from one old language — Latin. Hydro is my name from another — Greek. Different spellings, different sounds... but I carry the exact same thing into a word: water."

To prove it, Aqua flowed through word after word. As aqua, it made aquarium (a water-tank), aquatic (living in water), aqueduct (a water-carrier). Then it slid into its other coat, hydro, and made hydrant (a water-tap), hydrate (to add water), dehydrate (to take water out).

Six different words. One meaning underneath all of them: water.

"This is a gift and a small trap," Aqua explained. "The gift: if you learn both my names, you double the words you can crack. See 'aqua' — water. See 'hydro' — also water. The trap: don't expect a meaning to always wear the same spelling. The same idea can come from different old languages and look totally different on the surface."

The creatures started collecting Aqua's two faces. Every time they met "aqua" or "hydro," they knew, instantly, the word had something to do with water — even a long, science-y word they'd never seen.

Aqua made one honest correction. A young creature saw "hydro" and shouted "water!" — but the word was about anger (an unrelated look-alike part). Aqua rippled gently. "Check that it's really me and not a look-alike. Most 'hydro' words are mine — but read the whole word to be sure. A clue is a strong hint, not a guarantee." Confirm, don't assume.