This is the 16014th most frequent Russian word.
пузом
"Пузом" means "with/by the belly" in English.
Here, 'пузом' indicates the instrument by which the bear is positioned, describing the direction the bear's belly faces.
Медведь упал на землю пузом вверх.
The bear fell to the ground belly up.
In this sentence, 'пузом' describes the position or manner of resting, specifying that the belly is in contact with the sand.
Он лежал на пляже пузом на песке.
He lay on the beach belly down on the sand.
Here, 'пузом' refers to the orientation of the child's belly while swimming, indicating the downward direction it faces in the water.