Help.

Using PP 2007, I want to construct a page of bullets so that the items are emphasized one by one as I click through. By this I mean:

I want to start with a blank screen (except for the header)
As I click I want each item to be added one by one (so far so good)
At the next click I want the next one to appear (also easy)
and (here's the hard part) I want the previous line to fade partially but not completely (go dim).

I can get the previous line to fade totally away, but not just go dim.