Are you Two or Four??

Saturday we were driving in the car. Matt was talking to me about something random.  I can't even remember what and Arlo pipes in and says," Daddy, why?"

Matt responds, "I don't know why. Why do run around the house and destroy things?"

Arlo is quiet for a few moments, then says, "Mmmmm, ahhhhhh, wanna touch, Daddy."

"Why?" is the latest word/question/phrase and we're hearing it all the time. He'll even ask Sam and Sam's common response is, "I can't know why, Arlo."