Just think
of a plum.
Why the drool?

The full narration of the video.

Picture a green plum. Is your mouth already getting wet? Some eighteen hundred years ago, Cao Cao was marching his army. There was no water, and his soldiers were too thirsty to go on. He pointed ahead and shouted: "There's a big plum grove up there, and the plums are sweet and sour!"

Hearing that, every soldier's mouth began to water. That's the old saying: gazing at plums to quench thirst. There wasn't a single plum. So where did the saliva come from? Let's follow the word "plum" and see where it goes. First, where does saliva come from? Inside your cheeks, under your jaw and under your tongue, three pairs of little factories are hiding. They're called salivary glands.

Together they make half a liter to a liter and a half a day, about one to three bottles of water. Most of the time they just trickle. The moment food hits your mouth, they go full speed. Nothing pushes them harder than sour. When acid touches your tongue, saliva rushes in, washing the acid down and protecting your teeth.

But this time, there's nothing in your mouth. So who gave the order? The word "plum" slips in through your ear and reaches your brain. The word itself isn't sour at all. To someone who has never eaten a plum, it's just a word.

But you have. Your brain remembered that sour bite. It's like the lunch bell at school. A bell isn't food. But after years of it, the moment it rings, you want to run to the cafeteria. Your brain did the same thing: it tied "plum" and "sour" together.

When the word arrives, your brain thinks sour is on its way. It tells the salivary glands ahead of time: get the saliva ready. The order runs down the nerves to your cheeks and under your tongue. No plum in sight, and the saliva is already here. A learned response like this has a name: a conditioned reflex.

The word "plum" is the lunch bell. Watering at the mouth is the run to the cafeteria. The difference: you can hear the bell and choose not to run. Saliva doesn't ask you. It just comes. The first person to really see this clearly was Ivan Pavlov, a scientist who studied dogs.

Every time before feeding his dogs, he played them the same sound. After a while, before any food appeared, the sound alone made the dogs drool. You thinking of a plum and that dog hearing the sound are the same thing.

It's stronger in some people than others. The more real it feels, the more saliva you make. Smelling one, or watching someone bite into one, works better than just saying the word. Back to Cao Cao. Can saliva really quench thirst? Not really. Saliva is water that was already in your body. Moving it into your mouth doesn't add a single drop.

But with wet mouths, the soldiers felt less thirsty, held on a little longer, and really did reach water. It wasn't the plums that fooled them. It was their own brains. How about you? What's the one thing that makes your mouth water just thinking about it?