Answer:
What actually causes sucking is the collisions that occur outside the system (e.g. bottle)
Explanation:
From the passage, we discover that the collisions that causes sucking or contraction actually occur in the outside surrounding of the system. These move to regions of lesser collisions per unit volume. Those particles that hit the outside of the particle crush the system. The collisions that occur at the inside of the system cause it to expand while the outside collisions cause to suck or contract.
