premise: A man excitedly pushing in lawn mower in the backyard of a house. hypothesis: A man is driving a car in the backyard.
contradiction: You cannot be pushing in lawn mower in the backyard of a house while also driving a car in the backyard.  Pushing "a" lawn mower is the proper term, not pushing "in" lawn mower.  It doesn't make sense to be driving a car in the backyard.