you can't actually do that.... Bryce doesn't let you animate the "hidden" property. Wouldn't that be nice? Unfortunately, this is a limitation of the Bryce animation engine, and really needs to be fixed in version 5. Here's the only solution I've found: when you want to "hide" an object, move it way off screen. Next, in the Advanced Motion Lab, set it for an immediate transition time, which looks like a vertical line. That's about as good as it gets.