In a sound wave, air is being compressed and decompressed. If sound is reflected at the end, then to create a standing wave the compressed layer of air coming back must coincide with another compressed layer of air going forward. If that's the case, how can the compressed airs continue to travel in their respective directions after they hit one another? Wouldn't they just stop moving once they hit each other? Same for the string. If the original wave starts pointing up, then the reflected wave points down. But wouldn't each point at which the original and reflected wave coincide pull on the same part of the string just as strongly but in opposite directions, and therefore the string would just be flat? In this image: http://imgur.com/a/1d2f3 The wave going to the right will meet the wave going to left. The right side of the right-traveling wave will go up, and the left side of the left-traveling wave will go down. When they meet, they will be pulling on the same parts of the string. So wouldn't the up and down forces cancel to give a string that's flat everywhere?