We were discussing the speed of sound in math today. My students were having problems with the idea that sound had a speed (after all, sound is pretty fast and it seems like, when you say something, the other person hears it instantaneously). So I used the example of thunder. Most people know that, the longer it takes for you to hear the thunder after a flash of lightning, the farther away the storm. Sound travels at around 750 miles per hour (light is significantly faster, at around 180,000 miles per second), so each 5 seconds or so between the flash and the crash translates to one mile away.
One of my boys appeared to wake up at that moment. "Wait a minute! You mean thunder is the sound of lightning?"
It never occurred to me that some of them hadn't made that connection. "Well, yes."
He looked at me stunned for a second or so (meaning his thoughts were 0.2 miles away), then said. "Huh. I never knew that."