I use so many applets, video clips and simulations that I sometimes lose track of which one to use when. This website makes things easier as I have taken the time to make a lesson plan for each lesson which includes all the resources that I have used so before each class I have a look to see what I used last time. Yesterday I was doing "the end of the universe" with my HL class and reintroduced myself to a nice little applet on the big bang from UBC. It simply but effectively shows how varying the density of the universe affects the rate of expansion. Maybe a new version could include Dark energy? As the universe expands a graph of radius vs time is plotted (In rather difficult to see blue) showing the difference between open, flat and closed models. We didn't hang around to see if critical density gave a static state at infinite time but it certainly seemed to go on for ever. On the same day I was introducing my first year class to the gravitational field. I made what I call "the most boring animation in the universe" with two 1kg masses separated by 1m. Absolutely nothing happens however its the same force that pulls all the masses back in the big bang simulation. Same thing different time frame.