I recently encountered this from both sides in my work. I was trying to get a particular single-threaded CPU program running in less than 1 microsecond. Light goes about 300m in that time. I remember reading around the same time that the Eiffel Tower is about 300m tall. So it stuck in my head that if someone switched a flashlight on at the bottom of the Eiffel Tower at the same time as my program starts the program should complete before someone at the top of the tower sees the light.