You must be twins

A Welsh school-inspector visiting a small village primary school was struck by the great resemblance between two small girls sitting at the front of the class.

"You must be twins", he exclaimed.

"No we are not", replied the little girls firmly.

"Well then," said the inspector to the first girl, "how old are you?"

"Nine years and two months."

"And how old are you?" he asked the other girl.

"Nine years and two months."

"Then you definitely must be twins."

"No we aren't, we're all that's left of triplets."