On Mid—autumn Day, Jill and her parents visit her grandparents.

Jill：Hello, Grandpa and Grandma!

Grandpa：Hello, Jill. Come in, please.

Dad：Let's have some mooncakes and watch the beautiful moon in the garden.

All：Great!

Dad：Jill, please come and help me.

Jill：OK…

Grandpa：I have a riddle for you, Jill. Sometimes it's a"C". Sometimes it's an "O". Sometimes you can see it, but sometimes you can't. What is it?

Jill：Is it "cloud"?

Grandpa：No. It's all "O" today.

Jill：Ah, it's the moon!

Grandpa：That's right! Look! It's big and bright in the sky.