Why?

"Come on, Nina, let's brush your teeth"
"... whhhyyyy?"
"Because we need to get them clean."
"why?"
"Because there's still a little bit of dinner on them, and they aren't clean yet"
"... whyyyyy?"
"Well, we need to clean them because clean teeth will grow up healthy, and help you have strong teeth when you're a grownup. Otherwise you might not have such good teeth..."
"...why?"
"Well ... um ... because ... little tiny tiny tiny bugs will grow on your teeth and hurt them" (wait: was that just a little creepy for a two-year-old on the verge of bedtime? whoops)
"...why?"
"Ummm ... because they can...?"