A hippo can open its mouth wide enough to fit a 4 foot tall child inside.

Hippos can indeed open their mouths extremely wide - up to 180 degrees, which is nearly a full vertical opening. An adult hippo's mouth can open wide enough to accommodate a 4-foot tall child or object. Their massive jaws are designed for combat and dominance displays, with tusks that can be over a foot long. This extreme mouth opening capacity is real.