To buy something

To “buy something” means to believe it or accept it as true. The other, more literal meaning of to “buy something” is to make a purchase, but that’s not what we’re talking about here. In the episode, not everyone is “buying” the evidence from the excavations at Chiquihuite Cave in Mexico, or not everyone believes it. If you tell your teacher that your dog ate your homework, she probably won’t “buy it,” or she probably won’t believe you. In response, the teacher might say, “I’m not buying that!”