Nearly every dog owner has been brought to their knees by their favorite canine's "puppy dog" eyes.
Now a study published Monday says evolution may be behind the famous facial expression.
According to researchers, a specific muscle causes dogs' inner eyebrows to raise, making their eyes appear bigger and more infant-like. They say dogs do this specifically to get a reaction out of humans.
Researchers say canines adapted the expression over 33,000 years ago, as they became domesticated.
And it's not the only mannerism dogs have picked up to melt our hearts.
The study notes that dogs are better than other animals at using communication cues like pointing gestures, gazes and keeping eye contact.