The Demo: A bright laser is shined through one end of a long, curled piece of plastic. The laser beam travels through the curves of the plastic tube, without escaping, until it reaches the end where it is re-emitted.
The Physics: This is a demonstration of total internal reflection. For total internal reflection to occur two conditions need to be true: firstly, the light must be attempting to travel from a denser medium into a less dense medium. In this case, the laser is attempting to travel from a dense plastic into a diffuse gas (air). The second condition is that the angle at which the light is striking the boundary of the the two mediums must be greater than the critical angle. If the curved plastic coil made a sharp right angle, the laser beam would have no trouble escaping from the side of the coil. However, the curvature of the plastic coil is small enough to keep the beam trapped inside until, that is, the beam reaches the end of the tube. Here, the beam makes a right angle with the end of the tube so the beam is passes right through.
This basic principle of total internal reflection has a very practical modern application. Fiber optic cables transport information in the form of light rather than electricity. In general, fiber optic cables transfer a greater amount of data that their copper counterparts.