How does this work? Floors add to building on mouseover... 2 story becomes, 4,7, etc

I know I’ve been buggin you guys with questions about colorsmagazine.com flash sites… sorry! here is another one…

How does this work?

www.colorsmagazine.com

click archive
then click #55, birmingham
then click “family life”
it is the first image, roll your mouse up and down to reveal and hide new building floors.

Thank you! :slight_smile: