How Many Letters?

(Tom and his father are at home.)

Father: How many letters are there in the English Alphabet(字母表)?

Tom: Sorry, I don't know(知道).

Father: What? You are in school for one year.

Tom: Yes, but you are in the post office(邮局) for eighteen years. How many letters are there in the post office?

Father: Sorry. I don't know.

Tom: Haha. You are not a good postman.