I am

The English teacher says, "Mary, I'd like you to give me a sentence beginning with 'I', please."

Mary thinks for a few seconds and then says, "I is..."

The teacher interrupts her, "No Mary, you cannot begin a sentence with 'I is' - you must use 'I am'."

"But Miss..." Mary tries to say.

The teacher shouts, "Give me a sentence beginning with 'I am', please."

Mary shruggs her shoulders and says, "I am the ninth letter of the alphabet."