Chaotic Conversations

Been having quite a few of these lately. Engaging Bob in meaningful conversation is about impossible. He doesnt follow my conversation and often blurts out things that have absolutely nothing to do with what we are or were talking about.
Today at lunch I asked him if he was really hungry or if he just wanted a "light" lunch.. He responded with a huge laugh and said wouldn't that be like letting a monkey trim your nose hair?
Huh? Lost me!