Even if you move it with a pencil it goes back to its original shape
In: 6
The other answers have some really good technical details, but I think are missing an important part of it that I think helps get an intuitive sense for it. Any system when isolated from outside interference will eventually find a point of equilibrium. Some examples:
* Water will flow to the lowest place it’s able, and then stop
* A hot object will eventually become the same temperature as its surroundings, and then stop
* A spring that’s compressed will expand, then oscillate back and forth until it comes to rest
What all of these systems have in common is that they will, when left on their own, come to a state with a minimum amount of energy in it. Another way to phrase that is the energy that’s in the system can’t do work anymore:
* Water that’s high can be used to drive a water-wheel, but if the water is already on the ground it can’t be used
* When there is a temperature difference between two things that difference can be turned into work (steam engines, thermocouples, etc). When there’s no difference in temperature you can’t get work out of them, even if they’re both hot.
* A spring that’s compressed or stretched can be used for work, but one that’s not, can’t be.
Other commenters have talked about surface tension, so I won’t explain how and why it exists, but I’ll add this: Because of the surface tension of water, a sphere is the shape that has the LEAST energy for the same mass of water. It is also the shape that has the LEAST surface area. A balloon is similar. You can squish it into a square, but it will always tend to move back towards a sphere when left alone. You could use the balloon like a spring and store energy in it by squishing it, and using it by driving a load when it moves back to a sphere. Any energy released from the system that isn’t being used for work will turn into waste heat. This means that when two droplets of water merge into a larger drop, the single, larger drop has a smaller surface area than the two individual drops, and the difference in energy will turn into a tiny amount of heat.
So, a more complete answer might look like this:
* Spheres have the smallest surface area for a given volume
* Surfaces under tension will try to form shapes with the smallest surface area
* Water’s molecular properties result in water having surface tension
* Therefore, water will tend to form spheres
The half-sphere is caused by gravity adding its own forces on the system. When comparing the forces caused by either gravity or surface tension, gravity will dominate on large objects, and surface tension will dominate in small objects. Therefore smaller droplets will be taller and bigger droplets will be flatter. Water in a weightless environment will form spheres no matter the size of the “drop” since there is no net gravitational force on it.
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