They open it very wide.
the hippo would win because it has a big mouth.
You can't. You can only open your mouth as wide as your jaw will let you.
The hippos mouth is 4 feet (1.2m) and its teeth are big too. See: Hippopotamus - The Hippo's Teeth And Its Diet for more information.
Open the mouth wide.
A grown hippo usually has a mouth about 4 feet tall.
Birds
A hippo has the bigger mouth. Tasmanian tigers were about dog-sized, while a hippo's mouth is about the length and width of a mid-sized suitcase.
roughly 65 degrees
A hippo's tooth is about 13 in. wide. :)
Wide open. A gaping hole is a wide open hole. A gaping mouth is one which is hanging open, usually from astonishment.
The very large mouth of the hippopotamus is used to scoop up large amounts of aquatic vegetation, that the hippopotamus eats.