A Soldier's Lie

Once a soldier asked his commanding officer for a day's leave to attend his sister's wedding.
The officer asked him to wait outside the door for a few minutes while he considered the request.
The officer then called the soldier back in and said, "You are a liar. I've just phoned your sister and she told me she's already married."
"Well, sir, you're an even bigger liar," the soldier replied, "because I don't even have a sister."