What
is so impressive about this trick is the speed with which the water
drains from one bottle to another once you've created a whirlpool.
Try
draining the water from one bottle to the other without spinning them –
you make very slow progress as both the air and water battle to get
through the small gap at the same time.
By
spinning the bottles you create a whirlpool which has a hole in the
middle of it, in fact you can look down onto your whirlpool and see a
tunnel right the way through the centre.
The
air can now travel up through this hole, while the water drains down
around the sides. As they don't have to compete for space, the process
is speeded up immensely.
There's
a myth that because of the Earth's rotation, water always goes down the
plughole in a clockwise direction in the northern hemisphere and
anticlockwise in the southern hemisphere.
The
Earth's rotation does affect the direction of circulation for large
bodies of fluid, such as atmospheric storms or oceanic currents, but in
your bathroom it's the shape of the basin, or the movement you generate
as you take the plug out that causes the water to spin in one direction
or the other.
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