What is the tightness across the surface of water called?
It is called surface tension.
Erosion is the process of Weathering and transport of solids.
Explain why if you pour water into a glass very carefully you can stack the water slightly above the rim of the glass?
it's called "surface tension". It is the property of the surface of a liquid which causes it to behave like an elastic sheet. This is why a "water strider" insect can skate across the surface of a pond without sinking into the water. It is also what causes a drop of water - the surface tension of the water pulls it into a ball.