Answer:
Scallops use jet propulsion to move from one place to another. Their shells make them denser than water, so they normally rest on the ocean floor. If a scallop wishes to remain stationary, hovering a fixed distance above the ocean floor, it must eject water downwards so that the thrust force on the scallop is upwards.
Explanation:
For every action, there is a reaction with the same magnitude but in the opposite way (third law of Newton), so when the scallop ejects water downwards, another force appears that pushes the scallop upwards.
