Sorry joke

At primary school I had an odd teacher who, if you did something wrong, would make you write a letter of apology to him. If it was really bad, you'd have to write several. 

I was new and didn't really understand the point when I did something wrong and was told to write five
letters of apology. 

Not understanding, I wrote "S O R R Y" 

He went balistic on me, of course, while I stood there wondering what his problem was. 

"But you said five letters! There are five letters!"