Final answer:
Water fills up the spaces between pebbles through the process of capillary action, which is driven by cohesive and adhesive forces. The adhesive forces between water molecules and the pebbles are greater than the adhesive forces between the water molecules themselves, causing the water to spread out and fill the spaces. Surface tension also helps to hold the water in place.
Step-by-step explanation:
Water fills up the spaces between pebbles through a process called capillary action. Capillary action is the result of the cohesive forces between water molecules and the adhesive forces between water molecules and the surface of the pebbles. These forces allow water to climb up against the force of gravity and fill the spaces between the pebbles.
When the pebbles are close together, the adhesive forces between the water molecules and the pebbles are greater than the adhesive forces between the water molecules themselves. This causes the water to spread out and fill the spaces between the pebbles.
Additionally, the surface tension of water, which is the result of the cohesive forces between water molecules, helps to hold the water in place between the pebbles.