@Mden 23 October 2021 more from that month

do you like intellectual pleasure? We have a smart machine that we call M. I will stand at a distance of 100 meters from the M. I'm going to ask M to throw a ball at me at 100 meters per second. Practically when I catches the ball it will have been one second since M throw it. And now I will ask M, to make a loud bang at me (and since the speed of sound = 340 meters per second) I'm going 340 meters away now. Practically when I hear the sound it will be one second since M make that sound. This explains the phenomenon of echo sound. When you make a sound in a valley, the sound will hit the barriers and return to you according to the distance of the barrier it hit. And now I will ask M to shine a strong light, and since the speed of light is 300,000 km per hour, I will go 300,000 km away,which is more than 7 times the distance between the Earth and the Sun. See you tomorrow when I reach that point.