Now listen to part of a lecture in a biology class.
Prairie dogs are small animals you're likely to find on wide, relatively flat, grassy areas of land in North America. They tend to live together in large numbers. Now, generally these animals don't come into contact with human beings, but every now and then, they do. So let's assume that some prairie dogs happen to live in an area where human beings frequently come and go.
Now the first time the animals would see a human being, they'd instinctively react by making a sharp barking sound like a dog and jumping up and down, essentially warning or alerting other prairie dogs that are nearby of this potential threat, exactly the same way they'd react if they came upon or were threatened by a snake or a hawk, an animal that preys on them. Their instinctive reaction would be one of fear. They'd keep an eye on the human beings until these scary, possibly threatening individuals are gone. And...they're likely to react this way every time they see a human being.
However, if people pass through the area day after day without threatening them or trying to harm them, well, it turns out that the prairie dogs will gradually stop barking and jumping up and down when they see a human being passing through the area. At some point, they'd stop reacting to humans as though they were a threat.