Randomness

hi,
i am making a game at the moment for a mc charactor to go around the screen (by the mouse) and catch random mc’s falling down the screen. ive got it working apart from one thing. ive got a man as a cursor and when he hits 1 of 2 objects the score goes up or down, and ive got a health bar including actionscript for winning or losing the game. but my problem is, is that i dont no how to get the mc’s to go down the screen randomly and multiply randomly so that the mc’s falling will never end.
thx a lot if u can post a reply