How to add a property to a moviecip/class?

programming flash
1

So i have a movieclip on stage and I addChild(myMovieClip).
I need all the movieclips to have a number (like the first one i put with be 1, the second 2 and so forth)
How can I add to the movie clips custom properties like myMovieClip.mynumber
then give them their numbers when i add them to the stage

addChild(myMovieClip)
myMovieClip.mynumber = 56;