Really useful if you want to play around with their code and share it with your kids, etc. http://jsfiddle.net/yr64fy2o/1/ http://jsfiddle.net/yr64fy2o/1/ It

Really useful if you want to play around with their code and share it with your kids, etc. http://jsfiddle.net/yr64fy2o/1/ http://jsfiddle.net/yr64fy2o/1/ It is nice to be able to show someone directly how changing the various parameters affects the simulation.