Even thinking about yawning is enough. In fact, there's a 50% that you're about to, or just did, yawn.
Research shows that 11-week-old fetuses yawn. Yawns become contagious to people between the first and second years of life. When one person in a group yawns, over half of the people in the group will yawn within 5 minutes, and the rest will at least be tempted to yawn. Even animals yawn when they see another animal yawn. The average duration of a yawn is about 6 seconds, and guys yawn for longer than girls.
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