根据短文回答问题：How did P.E teacher know the truth?

根据短文回答问题：How did P.E teacher know the truth?
Dear Lucy,
Why are you always very angry with Ann?I think you should stop it now.
You think she really wanted the P.E.teacher to know that you weren't really sick yesterday,and that you went home just to study for your math test the next week.But let me tell you:this is not true.Ann is an honest (诚实的) girl.If you don't want Ann to tell anyone your secret,you shouldn't tell her the secret at
all.
In fact,Ann really didn't know what to say when the teacher asked her where you were.When she finally told the teacher "I think she felt sick and went home",her face was all red.The teacher,of course',knew Ann wasn't telling the truth (说实话).So he asked Ann,"Is that really true?" Then Ann just looked down and said
nothing.
That's how the teacher knew you weren't sick yesterday.So the truth is:Ann did her best to be your friend.
I think you should forgive Ann and stop acting like a baby.Before you went home,you should have let the teacher know first.It was not right to ask your friend to lie for you.I think you should tell Ann you are sorry for asking her to do something wrong.
Just think about this:do you really want to lose a friend because of something silly?
Love,
Linda

How did P.E teacher know the truth?
answer: The teacher knew it by seeing her face turned red when he asked Ann where she was yesterday, and seeing she just looked down and said nothing when he asked her whether it was true.