Dialogue - review "Present Continuous"

Billy: Hi, girls! What are you doing?
Girl 1: Hi, Billy! We´re writing a letter.
Billy: Oh, yeah? And who are you writing to?
Girl 2: Sadako. We´re writing to Sadako.
Billy: You mean you´re writing to Sadako Sasaki? But she died many years ago!!!
Girl 2: We know that.
Girl 1: We are not writing to Sadako herself. We´re writing to the Peace Memorial Park in Hiroshima.
Billy: And what are you writing?
Girl 2: Just a message of peace.
Girl 1: And we´re going to send one thousand paper cranes together with the letter.
Billy: Did you make all the paper cranes?
Girl 1: Not yet! But everybody in our class is helping.
Girl 2 : Do you want to help us? We still have to make 200 paper cranes.
Billy: Yes. Why not? It´s going to be fun.
Girl 2: Now, take one of those square papers and look at the instructions. It´s easy to fold a paper crane.
Billy: You´re right. it´s easy to fold a paper crane. Can I fold another one?
Girl 1: Sure. You can fold one hundred and ninety-nine cranes if you want.
Billy: All right! I´ll try.