It might seem like a TV trope, but there’s actual science at play here. It works like this: The sight of a uniformed stranger carrying a big, goofy bag drives Fido to get excited and bark. When the letter carrier leaves to visit the next mailbox on his or her appointed rounds, the dog mistakenly believes yelping drove them away. So the next day, it barks again. And it works again! And again.

It’s a case of operant conditioning: The dog acts that way because the behavior is reinforced and rewarded—the postal service is unintentionally training your dog to go bonkers. And it’s a huge issue. Aggressive dogs are such a constant (and costly) problem that the USPS recommends all postal workers carry a full can of dog pepper spray, which stains the “dog menace” yellow so it can be identified.

To alert mail carriers of toothy pooches along a route, the agency issues bright orange “Dog Warning Cards,” which reveal the dog’s address, breed, and name. But even that’s not enough. The USPS still releases a yearly dog attack report, which recently revealed that hounds took a bite out of 5767 postal workers last year. That’s nearly 16 attacks a day. With that in mind, maybe you should throw your mail carrier a bone.