Fallon explained to the audience how his producers would flash images of kids -- making crazy comical expressions -- across the screen, and the two would attempt to recreate the facial contortions, live.
Don Hemingway star Law showed off his impressive acting chops during the segment by mimicking two little girls: one attempting an adorable fishy face, the other little one pulling a very sudden, cross-eyed expression with her tongue sticking out to the side.
Unable to contain their laughter throughout the segment, both he and Fallon concluded their back-and-forth with a grand finale from a kid named Ian, whose smushed, duck-lipped, squinty-eyed, contorted expression was a hard to recreate.
In Fallon's defense, Law has years of experience mastering the funny face as father to four kids. Hopefully the Tonight Show host will eventually catch up, as the new dad to baby girl Winnie Rose, 7 months. "She's so cute," he gushed of his newborn daughter, after he and wife Nancy Juvonen welcomed Winnie last July.
Watch it here!