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.
A python can open its mouth very wide to swallow prey, stretching up to four times the diameter of its own body.
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.