It's wrong to make someone scared.
Doug knows that his sister will be coming home from school in a minute.
Doug wants to greet his sister.
Doug makes smoothies for his sister and himself and surprises her with it when she gets home.
Doug hides behind the door wearing a scary face mask, jumps out, and scares his sister when she get home.
Doug's sister is happy to get the smoothie and spends time hanging out with her brother while they drink the smoothies.
Doug's sister is mad because he scared her.