When your pooch gazes up at you with those adorable puppy-dog eyes, it can be hard to say no. That may be why dogs learned to give that look, scientists say. New research suggests that canines developed extra fast-twitch facial muscles to win over humans. 

Dogs descended from wolves thousands of years ago. As they became domesticated, they gradually started to make these endearing expressions, say researchers at Duquesne University in Pennsylvania.

Today’s dogs have more fast-twitch muscles near their eyes and mouths than wolves. The adaptation makes sense, researchers say. Dogs use the muscles to communicate through barking and those irresistible eyes. Now hand over that treat!