Why do we yawn?
Nobody knows for sure. A leading idea is that yawning helps cool the brain, but scientists still debate it. Why yawns are contagious is a mystery too.
Going deeper
The old explanation, that yawning brings in extra oxygen, hasn't held up: breathing more oxygen doesn't make people yawn less. One popular alternative is that a big yawn pulls in cooler air and boosts blood flow, cooling the brain slightly.
Contagious yawning is even stranger. Seeing, hearing or even reading about a yawn can trigger one. Some research suggests we catch yawns more easily from people we're close to, but the evidence is mixed.