Jason always asks interesting questions, today he wanted to know if the age of the universe was twice as old as it is now wouldn't the Hubble constant be the same? His reasoning was that if space was expanding at a constant rate then the galaxies at say 50Mpc will be travelling away at the same rate as they are today therefore the graph of velocity vs distance would have the same gradient so the age of the universe would be the same. Explaining in terms of moving cars is easy enough since the car that would be at 50Mpc in a universe twice as old as today will be travelling at half the speed so the gradient of the graph would be less. Explaining in terms of an expanding universe is more difficult. I tried stretching a spring to show how the different parts move at different speeds but it wasn't convincing so I tried it with interactive physics and got a nice result. Analysing the motion of the different coils in LoggerPro enables a velocity distance graph to be plotted that is very close to straight, the reciprocal of the gradient gives the time taken for the spring to reach the chosen length. I'm going to write a worksheet for this and get my class to do it next Tuesday.