Dew on the grass A large drop of dew forms on the very tip a blade of grass.
Dew forms on grass when a blade of grass looses heat by radiation from its whole surface and is cooled down to the dew point of the surrounding air. The whole surface is cooled down to the dew point.
Why is the dew not distributed evenly over the whole surface? 
 A: 
A large drop of dew forms on the very tip a blade of grass. Dew forms on grass when a blade of grass looses heat by radiation from its whole surface and is cooled down to the dew point of the surrounding air. The whole surface is cooled down to the dew point. Why is the dew not distributed evenly over the whole surface?

Water molecules are cohesive, in other words sometimes  they stick to each other more than they stick to another object.

If water molecules stuck to each other in preference to anything else, then drying yourself with a towel would be impossible. Some materials are hygroscopic, and some are hygrophobic. You have to consider the interaction of water molecules and the water-repelling leaf. Thank you to Suzu Hirose for pointing this out.


Those molecules at the surface of the water layer on the leaf lack water molecules surrounding them and they develop a tendency to cohere to water molecules below and beside them. This causes the water to pull itself into a ball/ spherical shape.   
A water drop occurs in the lowest potential energy state, where the mimimum energy is used to maintain the shape. As I stated above, this occurs when cohesive forces for the water pull it together. In a gravity field, such as on  Earth,  the sphere is pulled from a sphere into the drop shape we observe on the leaf. 

Images Sources: Water Drops on Leaves
