have you ever thrown a rock into a pond and wondered why the water moves in that ripple pattern? well i have, so i decided to figure out what makes that happen. i found that when you throw the rock into the still water, it creates a disturbance. this disturbance transfers energy and momentum into the water pushing it and making it ripple; its called a wave. the ripples in the water are a transverse wave. the particles in the water move in a direction perpendicular to the direction the wave is moving. if is a leaf sitting on top of the water and you throw a rock in, when the waves energy reaches the leaf, it will move up and down while the wave is still moving horizontally. it moves like this because it is a transverse wave.
so what happens when you throw two rocks into the water simultaneously? when both the rocks create a disturbance in the water, they will both make waves. when the waves collide, they will either create constructive or destructive interference. if a crest meets with a crest and a trough with a trough it will create constructive interference. when a crest meets a trough, it will create destructive interference and cancel out the motion of the waves leaving a still spot.