Water molecules are attracted to each other by what is known as cohesion, in this case hydrogen bonding. If the water droplet is on a waxy surface, such as this leaf, the hydrogen atoms in each water molecule are attracted to hydrogen atoms in other water molecules, rather than to the molecules that make up the substance surface. If the surface was non waxy, such as paper, the water molecules would have a greater attraction to the paper surface, this is called adhesion, and the water would just spread out.
All substances try to exist at their lowest energy state, and for this to happen with a water droplet, each water molecule wants to be surrounded by other water molecules, and when this happens, they naturally form a spherical shape. However, we also have to take into account gravity, which of course acts downwards, and thus creates a flattened sphere like ‘structure’. Obviously, the smaller the droplet, the more spherical it is, and once it gets over a certain size, gravity exceeds the water tension, and the droplet collapses.
If I have any of the physics wrong, which is fairly likely (LOL!! ☺), please don’t hesitate to let me know in the comments below.
For another great mathmatical post, see Tara’s blog post, “Natural Numbers”, all about the Fibonacci Sequence.