1, actually very simple, because water has a certain viscosity and tension. When the paper clip is put in, the water level rises. However, the water at the edge of the cup is kept connected because of the viscosity between water molecules (or the tension of the water surface), thus forming the phenomenon that the water level is higher than the edge of the cup.

2. The number of paper clips put in depends on the surface area of the cup mouth and the mineral content in the water, because it is the tension of the water that holds up the paper clips. The larger the surface area of the cup mouth, the more mineral content in the water, the more water molecules on the surface of the cup mouth, the stronger the cohesion of water molecules, and the greater the surface tension of water.

The water we usually see exists as a whole. Because the tension of water molecules and the gravitational repulsion between molecules are balanced, the paper clip put in has no destructive balance structure, so the water will not overflow. If you keep adding paper clips, water will flow out after a while.