A Japanese company called Langualess just created a harness for dogs with a sensor that changes color based on how the dog is feeling: Relaxed, excited, happy, interested, or stressed.
The harness connects to an app on your phone, so you can track your dog's moods over time.
The company says they figured out how to detect a dog's emotions by studying heart rate patterns for dogs in different situations.
The harnesses should go on sale in the U.S. sometime later this year. They're already on sale in Japan for around $37.