An often recurring issue in making an animation is the wish to make an object disappear or fade away at a certain frame, either for the sake of the effect itself or to replace the object by another one to achieve some dramatic impact (such as an explosion or a bunny rabbit changing into a ball).
There are many ways to engineer these effects, and most of them are not specifically tied to script links reacting on a frame change (many can simply be keyed as well). Nevertheless, we will look at two techniques that may easily be adapted to all sorts of situations, even ones that are not easily keyed. For example, we demand some specific behavior of a parameter that is easy to formulate in an expression but awkward to catch in an IPO.