every mc has the property "depth" so every mc has a "depth" ok?
even though the depth is 0, it still has a depth.
something that is transparent, has a color, but that color is transparent. It has a color, even thoug hyou can't see it, ok?
You cannot remove the mc if its depth value is smaller than 0. When you attach them dynamically it is a good idea to set their depth value to something positive (I'm not sure if it is poissible to set it to a negative number)
All MCs, Buttons that you create within the Flash IDE at authortime have a depth of below zero, that'S why you first have to swap their depths to something positive in order to being able to delete them.