Why does a drop of water form a circular pattern on a horizontal table? Why not square or oval?

First of all, water droplets will be tiled on the desktop to the greatest extent under the action of gravity.

Then, there is intermolecular force (van der Waals force, non-hydrogen bond) between molecules on the water surface. Note: Hydrogen bonds only exist in the solid form of ice or water. ) Extend outward and accumulate inward.

At the same time, the circular pattern is symmetrical from all angles, so the force is balanced. However, a square and an ellipse are only symmetrical at a certain angle, so the force is unbalanced. So it's most likely round. (especially for very small water droplets)

Plasticine is different because water is liquid and plasticine is solid.

The falling of plasticine can be explained by the momentum theorem in high school. When plasticine falls from the sky with great momentum and touches the ground for a short time, it is crushed by the strong impact of the ground.

At the same time, it is round, so it becomes round.