Get a medkit, a vial of clear liquid (the brown glass container) and a backpack.
Place the medkit and the vial inside the backpack and make sure there is at least one more space left in the backpack.
Pull the medkit in your hands to open it. Then drag the syringe from inside the medkit onto the vial to get a usage dialog. Select "fill syringe with unknown liquid". Wait for server delay and a new syringe in pristine condition, filled with unknown liquid, will appear inside the backpack, leaving the original syringe inside the medkit.