Tidus sputters, as if he can't believe Inigo's question (as if he wants to laugh, but he's got to keep it back!), throwing the guy an incredulous look that's amused.

"What else would you be doing in there? You're growing!" He spreads out his hands like, duh! Then they start making a gathering motion. "You're aging up to the age of five, taking in the knowledge of how to be a normal person. You learn the alphabet, language, maths, how to walk and taaalk... it's the easiest way to do all that!"

Tidus looks to Inigo, face confident with brows up and mouth smiling, nodding to him. It makes sense, doesn't it?!