Quite simply, I was knocking at the door before you came. When you saw my hammer hitting the railing, you heard the last time I hit it. Light travels much faster than sound. When I raised the hammer, the sound of my knocking on the railing reached your ears again this time. I hit the railing twice at 680 meters and made two noises. The interval between my first strike and my second strike is 1 sec, and the sound comes out 340 meters. After the second blow, the sound spread 340 meters. After the third blow, the sound reached your ears. Only when I hit it, the sound came out. The process of my hit took 1 second, but there was no sound this second. It takes three seconds to knock twice when the sound reaches your ear. The sound of the first strike reached your ears, and I struck twice. The sound of the first strike reached 340 meters, and the sound of the second strike came out. The first sound reaches your ears, and the third sound reaches 340 meters. When I knocked again, the sound reached your ears for the third time. I don't know if you understand this cycle. It's simple. Use your head. Wait until later, even if you don't study, you will understand with the passage of time.