Final answer:
Pushing on the plunger of a syringe creates a pressure difference that causes medicine to flow into a patient through a hollow needle.
Step-by-step explanation:
When you push on the plunger of a syringe, it causes medicine to flow into a patient through a hollow hypodermic needle due to the principles of pressure and fluid dynamics. The movement of the plunger creates a pressure difference in the syringe, forcing the medicine to move from an area of higher pressure (the syringe) to an area of lower pressure (the patient's body). This pressure difference causes the medicine to flow through the hollow needle and into the patient's body.