Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Zero is the net force acting on an object when two equal forces are applied from opposite directions.
What is the net force acting on an object when two equal forces are applied from opposite directions?

The pupil is the name of the black opening in the eye that lets light enter.
What is the name of the black opening in the eye that lets light enter?

Delta is a triangular feature that is formed when a river empties into a large body of still water.
What triangular feature is formed when a river empties into a large body of still water?