Jonah has discovered toothpaste. That's what "thoothsath" means, I believe. I've always just brushed his teeth with water, as I do my own, but Daddy uses toothpaste. So here, Jonah is giving his theories on why one might need toothpaste at all.
Hey, as far as I'm concerned, it's as good a reason as any. I'm not a big believer in brushing to prevent tooth decay. I have far more faith in some good quality cod liver oil, thankssomuch.