Because every turn in the pipe increases the friction made by the water.
Why does a wagon-wheel turn around when the wagon moves?
Because the friction between the wheel and the earth, prevents the wheel from sliding along.
Why can a steam-engine draw a train of heavy cars?
Because the friction, between the wheels and the rails, causes the wheels to turn around instead of sliding; and thus the engine moves forward drawing the cars with it.
Why is sand sometimes put on the rails?
Sand is put on the rails to increase the friction between the wheels and the rails, so that the engine may start a heavier train.
Why do we use sleighs when there is snow on the ground?
Because sleighs slip easily on the snow.
Why do sleighs slip easily on the snow?