Can't figure out how to a transition

I’ve seen this effect in a few places before.
You have one graphic.
Mini squares will ‘peel’ off this graphic and reveal another one below.
(Then the process is repeated… the next layer will peel off in squares and so on.)

I’m sure you do this with masks and actionscript.

But I’m having trouble thinking of where to start on this one!

Any help would be appreciated.