"Just Why"

Sam is in the "why" phase. Very much so, in fact. The other day while driving to day care ('school') we had (initially) a typical exchange:
"Daddy why are we going this way?"
"Because we're going to school."
"Why?"
"So you can play with your friends."
"Why?"
"Because you like to play with your friends."
"Why?"
"I don't know sweetie, why do you think?"
[long pause]
[shrug]
"Just why, daddy. Just why."
[confusion]
[Aha!]
"You mean 'just because?'"