When a pebble drops on the surface of water it disturbs the water molecules that it lands on, forcing them to be pushed away from it. This creates a wave on the surface since all water molecules are connected to each other through strong inter-molecular bonds called “hydrogen bonds”. You can imagine these bonds as springs between molecules. Therefore if one moves, it pulls and pushes the others around it. Overall a kind of “mexican wave” is performed which spreads in all directions, causing the ripples in the water’s surface.