Looking for a hack - do you know which one this is?

[LadyBeth]

Hello all,
One of our users had visited a site as a guest, and had read a few threads. He got a message that said something like "you have read 8 threads why not register?" or something like that. We have the one that encourages guests to register overall, but I like the idea of one that knows how many threads a guest reads and gives them a personal nudge. Any help, greatly appreciated as always.

LB~