Lost Parts
The trick is to have a good ear and remember the sound the part made as it dropped, bounced, and rolled. Then, you take an identical spare and drop it in the same way and whatch where it ends up, making sure it sounds the same as it goes.
If you don't know what small part fell off, you have to try several options, and choose the one that makes the right sound...
(Of course, if you already have a stack of spares, why would you bother about the missing part?)