Even if you move it with a pencil it goes back to its original shape
In: 6
Water has an invisible skin called “surface tension”. Think about what a partially filled water balloon would look like, and act, just sitting there. That’s what the water droplets are doing because of their surface tension. In essence, droplets of water are little water balloons that haven’t popped.
The surface tension answer is correct, but I feel like it just pushes the question back to “where does surface tension come from”.
Water is *cohesive* – i.e., water molecules like to stick to other water molecules, more than they want to stick to e.g. your pencil. There is an attractive force pulling the water molecules together into a blob and *holding* them together. This force is really the [electrostatic force](https://en.wikipedia.org/wiki/Coulomb%27s_law), the same “opposites attract” force between protons and electrons. Water molecules are shaped in such a way that the electrons mostly bunch up on one side of the molecule, leaving the molecule with a positive side and a negative side. Then in your water droplet the water molecules all orient themselves so that the negative side of one molecule is being tugged on by the positive side of another one.
A sphere has the least surface area for any given volume, and therefore, for the water droplet, the least interaction with other things that aren’t other water molecules.
Water is made up of H2O … That’s actually two positive hydrogen atoms (H+) bonded to a negative oxygen atom (O-)
Due to electromagnetic forces, the hydrogens are sort of “flanking” the oxygen so they aren’t a straight line like this:
H
O
H
They are more in an angle like this:
H
O
H
That makes the “left” side in this diagram have a more positive charge and the “right” side have a more negative charge:
+H
O –
+H
Because water is polarized like this, it acts like little magnets and likes to stick together. Water is honestly so cool.
Also this is why when water freezes, it expands. Because instead of flowing and bouncing around, all the little magnets line up and create little gaps between each angle >>>>>
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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