How does facebook place a header on link pages?

[vizulefllry]

When you click on a link someone has posted on facebook, it comes up with that page, however the web address is a facebook address, and there is a facebook bar at the top. Then when you click x on the bar it refreshes to the proper link and reloads the page. How is this done?

[Nigel A]

Without looking- It doesn't open the link, it opens a page of its own which contains an iframe. The content at the top is theirs, the iframe is the page someone was linking to. In the top bit, they've put the direct link to the site, which is the X.