if you read the manual well youll see that you need to enable collision detection for every object you want it to effect.(do this in the properties tab) secondly, you wont see any reaction if the objects are keyed to intersect eachother. itll only work if they inadvertently collide, as the resuly of gravity, another expression(python script i think its called). i messed around with it for a while and figured it out, but then never used it in anything, so i kinda forgot. but its more complicated than it needs to be, i think..... ganbatte!!!