[Newbies] Orphaned objects

Hi, guys:

My son has been programming a game in Morphic and has come across the following problem.

He wants to use allInstances to do stuff, like so:

myObjectClass allInstances do: [:o | o doStuff].

The problem he's having is that he's got orphaned objects from experimenting, and he can't get rid of them. They're sketchmorphs. If I add them to the world, they'll show up, and I can then delete them with the "X" but they simply vanish from the world, while persisting.

How do I find out what is referring to these objects and thus keeping them alive?

===Blake===