Short answer Yawns don't just spread between people. They can spread to other animals too, like dogs.
When you see someone yawn, you yawn too without meaning to.
You yawned just reading this, didn't you?
Contagious yawna yawn you catch from seeing someone else yawn
And it's not just a human thing. It's been seen in chimpanzees, dogs, cats and birds too.
It can even jump between species. When a person yawns, a dog will sometimes yawn too.
There's still no clear answer for why yawns spread. One idea is that it makes the whole group perk up together. That's just one of several theories.
Time to run a yawn test on my dog
Watch it as a short
Sources
Photo credits
More useless facts
PsychologyRepeat a Word and Its Meaning DisappearsSay one word over and over and something strange happens
FoodKetchup Used to Be Fish SauceThe word ketchup came from a name for pickled fish brine
GeographyIn Antarctica, Even the Time Zone Follows the Delivery GuyAt the very bottom of the Earth, the South Pole station's clocks are set to New Zealand time
MusicDo-Re-Mi Used to Be Ut-Re-MiThe do re mi fa sol la ti from music class started with ut instead of do