Math.random for Whack-a-mole

I’m trying to make a Whack-A-Mole type game except with my friend’s head popping up…basically you have to hit him before he goes back down.
The problem i’m having is that I don’t know how to make the heads randomly pop up…I have this statement on the first frame:

pop = Math.random();

And then for each head (a movie clip within a button) I have: (it varies with each head)

if(pop <.1)


if(pop <.2 && pop >.1)

and etc. for the different ones.

For some reason it doesn’t work…and I’m not sure if pop renews everytime it enters the frame, or what…if I have a stop(); statement on the first (and only) frame, will it continue to renew the value of ‘pop’ or will it just do it on the first time it enters the frame?

If I have made any sense…help me please! :frowning: