In the water
siphon the water will flow out of the end of the tube A. (Figure 1). If a hole is made in the tube
at P air will get in and the links between the water molecules will be broken and the flow will
stop.
A siphon is useful for emptying ponds, water tanks etc. where you need to
make the water 'flow uphill' at the start!
You can compare this with the movement of
a chain over a pulley (Figure 2).