This is the activity no. 3 in unit 3: module 1 right? Here is my answer before:
We were able to push the plunger with air in it but with water, we can't. It's because we felt the resistance of the water to being compressed....
Can you please write the activity, unit, and module number next time? It will be hard for the others to answer because they don't know what you are asking.
--Mizu