Smart hungry

A: Do you know the kind of bread?
B: A bean-jam bun, a custard bun, a half-melon shaped bun.
A: A half-melon shaped bun is different from two others.
     Do you know why?
B: Let's see. I don't find out.
A: Because it is shaped.
B: I see. It is difficult for me to solve.
A: LaLaLa. How do you like it's size?
B: Normal is best.
A: Me, too.
B: It get time to sleep. Good night.