answer: #Person1#'s back early because #Person1#'s teacher had a sudden stomachache. Mum's cooking for Grandma Wang because she's sick and her only daughter went abroad. #Person1# offers to help.
documents: #Person1#: Mom, I'm starving.
  #Person2#: Here are some biscuits. Why are you back so early today?
  #Person1#: My teacher had a sudden stomachache, so the class was cut shot. You?
  #Person2#: Me what?
  #Person1#: You are cooking at least two hours earlier than the usual.
  #Person2#: It's not for us.
  #Person1#: Then it's for Dad, isn't it? It's so unfair!
  #Person2#: Don't be a smarty-pants. It's for Grandma Wang.
  #Person1#: What was that again?
  #Person2#: It's for Grandma Wang. She is sick and her only daughter went abroad weeks ago. So she needs our help.
  #Person1#: I'm sorry, I didn't know that. But I wanna help.
  #Person2#: Umm, let me think for a moment. We can meet her together after I finish cooking.
  #Person1#: I'll get knee to knee with her.
  #Person2#: Good boy. I can only imagine how happy she will be to see you.