But we do know that even thinking about yawning, or listening to this story, can make you yawn. One theory claimed you yawned if you had too little oxygen in your blood. Sometimes you yawn if you are tired, but highly-alert athletes often yawn just before a big event. Soldiers will often yawn just before an exercise. People are most likely to yawn in that first hour after they've woken up, or in that last hour before they go to sleep, or when they're tired, bored, or doing long boring repetitive tasks.
People riding on fairly empty trains late in the day tend to yawn a lot.
If you smile, you'll probably get a few people to smile back, but yawn, and the whole room yawns with you. And if you don't believe me, ask yourself if you've yawned at least once during this story. And if you haven't, thank you for being so interested, and good night.