阅读下面的短文，选择合适的答案回答问题。

    Sam is very good at school, but he is not clean. His face and hands are often dirty (脏). He washes them with water only. He does not use soap (肥皂). Sam has an uncle. One day his uncle goes to see him. He looks at him and says, "Sam, I know what you ate for breakfast this morning." "What?" Sam looks at his uncle and says. "Eggs" says his uncle. "Your face and mouth tell me that." "You are wrong, uncle!" says Sam. "Not today or yesterday, but the day before yesterday!"