The idea here is to show that the pressure at the top of a siphon is below atmospheric pressure.
Start with a water-filled tube and create a siphon and move flow from one cup to another. What will
happen when you puncture a small hole at the top of the tube?
Since the bend of the siphon is above the surface of the water, its static pressure must be lower
than atmospheric pressure (proportional to the weight of the water column). Therefore, when the
pin makes a hole in the tube, higher pressure air will go into the tube instead of the lower pressure water coming out.