Riddle:
Two sentries were on duty outside a barracks. One faced up the road to watch for anyone approaching from the North. The other looked down the road to see if anyone approached from the South. Suddenly one of them said to the other, "Why are you smiling?" How did he know that his companion was smiling?
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Answer:
Although the guards were looking in opposite directions, they were not back to back. They were facing each other.