Marmaduke is riding a skateboard. While this in and of itself is absolutely HYSTERICAL (dogs aren't supposed to be able to ride skateboards!), the gag is kicked into overdrive when it is revealed (via his owner-boy's exposition to his fellow tricycle enthusiast) that not only is Marmaduke a dog who can ride a skateboard, he is a dog that is phenomenally skilled at riding a skateboard. You could cut the comedic absurdity with a knife.