How do you...

...make that your sig is different on each refresh?

Like I thought I was going nuts when everytime I see a post by Vex its got a different sig, even if I just read a thread and it had a different one. I'm sitting there thinking like does she change her sig every 5 mins or something?! lol. But I heard somewhere that you can have it change with every refresh or whatever, how do you do that?