Happy Birthday! — 英语阅读题

阅读短文，回答问题。 Today is Joe's birthday. Joe is nine years old. In the morning, his sister Mary makes a card for him. The card is blue. Mary writes some words on the card: "Happy Birthday!" Joe reads the card and smiles. He says, "Thank you, Mary! I like the card very much." Then they go to school together. At school, Joe's friends say "Happy Birthday!" to him, too. Joe is very happy today. 问题：What does Mary write on the card for Joe?（Mary 在卡片上写了什么？）请用短文中的原句回答。