How Many Letters Are There In The Post Office?

A father asks his son,"How many letters are there in the Alphabet?" "I do-n't know."says his son.His father says,"You don't know? You are in school for  one year and you don't know how many letters there are in the Alphabet!"he saysangrily.

His son says,"No.But let me ask you a question, Dad.You often go to the post office.Please tell me how many letters are there in the post office?"