Answer :
The substance I think is in the cylinder that you cannot see and prevents the plunger from going all the way in even when you push the plunger hard is Air.
What do you feel as when one push the plunger?
When a person push on the plunger, the person can feel the air pushing it back. When a person has stop pushing, the air inside the syringe will tend to return to its normal size.
Note that there are a lot of ways to compress air. A person can do so by squeezing a specified volume of air into a smaller space.
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