Popcorn Head - Part One

Kuro's classmate Aona is troubled by the popcorn that keeps popping out of his head. Aona says it makes him sad when the popcorn disappears, so Ramune lends him a bag that can be used to store the popcorn. Why is Aona's head producing popcorn, and why does he feel sad when it disappears? Ramune tells Aona that the treatment method will become clear if he asks himself those questions...