Hi, I would like to know what is the safest way to show an AD before content if the user comes from search engines, it works for IGN.com, they use the same URL but they show an AD for 20seconds first and then the page reloads with content, even the source code when the AD is running is the same as the content page.

There is any penalty on doing this?
Do you know any reference on this matter?
What type of code I will need?

Thank you

I didn't check out IGN but the way I do stuff like this is put the special content in a div, then have the server check the header for referrer=whatever and expose the div based on that. You can have a timer on it to go away after x seconds.

As far as penalty - the rule of thumb is to do for googlebot whatever you do for joe user. You could say that is exactly what is happening...if googlebot was referred by a google search then it would see the ad therefore doing this is legit, but as a practical matter there is risk.