Algodoo

I'm not certain if this is exactly what you want the box to do, but this simple script will make the size pulse between large and small, sort of like a heartbeat: size = [0.2, 0.1] * sin(sim.time)

If that is not what you want, then please explain in detail exactly what you would like the box to do.

I guess that's what you want. But keep in mind that the sine function goes from -1 to 1 so you're making the box go from positive size to negative size (and it disappears when it has negative size). Also if for some reason the sin(sim.time) gives 0, the box will get deleted by Algodoo

You have to do this:

size = [0.2, 0.1] * (sin(sim.time)+1.1)

This will make the sine function be between 0.1 and 2.1, so it will never be 0 (or negative for that matter).

Your problem was that you treated an array of numbers like a sum:
[2,1] is an array, while

[2+1] = [3] which is an array with 1 number (yes, it still is an array)

Please consider using math.sin instead of mere sin since it often happens that the algodoo config changes when loading scene, which often disables people like me from using sin in code. Math.sin is error-proof, while sin has the chance of not working on some systems.