The little brother of an Army radar operator asked，“Jim，tell me how does a radar work？”

“The radar transmitter emits brief impulses of electromagnetic waves which are reflected from the target and received by a special receiver．Since the speed at which electromagnetic waves propagate is exactly known and the time they take to travel to the target and back can be determined with a great degree of accuracy，it is possible to determine the range to the target as well as the direction to it．”

His brother pondered a moment，then said，“As long as you don’t know，Jim，why don’t you just say so？”