I tell them: (first, explain spacetime) "Relative to anyone else, we move through space-time at the speed of light. Take you and I standing here, you are not moving in space relative me so I see you moving through time at the speed of light. If somebody flew past at near the speed of light, I wouldn't see them moving through time very much at all. Since space velocity and time velocity always combine to the same total, people who are moving through space relative to you, move slower through time relative to you." In my opinion, a scientific theory shouldbe as sensible and accessible as possible to everyone. I dislike books that play up SR as some mystical time space weirdness. If any one has a better way to explain SR, or a way to explain length contraction in a nutshell, I would like to hear it.