The result is that the car gets jerked around to the right, off the road, even when you keep the wheels straight. Next I showed that light waves can be bent by air of different densities. You can't see the beam go across the room, but if you tap the blackboard erasers nearby, you can make a cloud of dust which will make the beam visible. Then I got out my propane blowtorch , and held the flame under where the laser light traveled, so that hot air rises and flows up through the beam. As the beam goes through pockets and bubbles of hot and cold air, it gets bent this way and that, which blurs and wiggles the laser spot visible on the wall.
Hot air expands, and therefore is thinner than cold air.
I asked the kids to imagine they lived in a small village, about a thousand years ago. Suppose someone had told them that the world is not infinitely big. Of course the answer in 1999 is that since the world is a sphere, it is not infinite, and yet you can go off in any direction, and never reach the end. In fact, you might come back, after traveling a long time, to the same village where you started, even if you were very careful to keep traveling in a straight line. All these things might have seemed mysterious and paradoxical to a 9th-century villager.