Physics
Answer
The syringe is kept with it's opening just inside a liquid and the plunger is lowered till it's base. Now when the plunger is pulled up in the barrel (as shown in figure), there is no air inside and thus the pressure inside the barrel below the plunger is much less than the atmospheric pressure acting on the liquid surface. As a result, the atmospheric pressure forces the liquid to rise up in the syringe.
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