Hi, (resurrecting a very old thread) You have your answer (recursion is supposed to work in reactive!) but I would observe that there are three solutions to defining a behaviour for the kind of model you propose: (1) Solve the problem completely so you have a purely time-parametrised behaviour. In general this involves solving d.e.s so it may not always be possible. It also doesn't support the interactivity well. I used the complete solution approach for the pong "game" (it's non interactive) in the crayon demo I just posted. (2) Solve the problem using mutual recursion between Events/Behaviours. This allows you to access the previous value "just before" the kind of instaneous change `switcher` introduces. (3) Approximate the problem using a boring finite clock-rate iterative simulation. This is the solution I used in my stars demo uploaded last december to this list. This is obviously the least elegant approach, but there are all kinds of algorithms much easier to express as iterative simulations than the other kind. You can use quite a slow iteration rate (I think I used 10fps in stars) and then interpolate between the 'keyframes'; it works very nicely. This is a good place to generate collision events etc. Jules