Z: I'm firsty.
Me: You're "firsty"? I think you're "thirsty."
Z: Yes, I'm firsty.
Me: Th, th, th, thirsty.
Me: You have to stick your tongue between your teeth, like this: Th, th, th.
Z: Th, th, th, thfirsty. Firsty.
Me: I'll get you a drink.