Toy designed for older audiences, such as teenagers or adults, demonstrate advanced concepts. Newton's cradle, a desk toy designed by Simon Prebble, demonstrates the conservation of momentum and energy. Not all toys are appropriate for all ages of children. Even some toys which are marketed for a specific age range can even harm the development of children in that range. "Age compression" is a toy industry term that describes the modern trend of children moving through play stages faster than they did in the past. Children have a desire to progress to more complex toys at a faster pace, girls in
Energy–momentum relation expressed in terms of wave quantities: and tidying up by dividing by throughout: This can also be derived from the magnitude of the four-wavevector in a similar way to the four-momentum above. Since the reduced Planck constant and the speed of light both appear and clutter this equation, this is where natural units are especially helpful. Normalizing them so that , we have: The velocity of a bradyon with the relativistic energy–momentum relation can never exceed . On the contrary, it is always greater than for a tachyon whose energy–momentum equation is By contrast, the hypothetical exotic matter has a