The set of the sails ...

A teacher said, "Mary, I'd like you to give me a sentence beginning
with 'I', please."

Mary thought for a few seconds and then said, "I is..."

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

Mary looked upset and said, "But Miss..."

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

Mary shrugged her shoulders and said, "I am the ninth letter of the alphabet."