premise: People are riding bicycles in the street, and they are all wearing helmets. hypothesis: A group of friends are grabbing their bikes, getting ready for the morning bike ride.
contradiction: You are either riding your bike or getting ready to ride, you can't be both riding and getting ready to ride at the same time.|contradiction: People riding bicycles in the street are not getting ready for a morning bike ride.|contradiction: People cannot be both riding and grabbing their bikes.