With PopUnder I mean a new window which open's under the window, like PopUp open's a new window on top of the window.

Hello guys,

I'd like to know hoe to do a PopUnder window (to an URL) so, that it starts only randomly. for example every 20'th time someone loads the page.
The frequency should be changeable, so it should be based on random numbers between 1 and n, so in case a visitor "get's" random number 20 for example, he'll get's a popunder window like "www.google.com ", but if the visitor get's number 1´-n (not 20),...