Feeding others RSS to create own RSS

[candyoff]

Hi

I once saw a website (Site A ) feed another site (Site B) RSS and create a new RSS.
By subscribe to the (Site A) RSS, I will see Site B RSS content but with a few sentences/advertisement under the new RSS.
I wish to know how he do that?
(And I fully understand its infringe copyright and just with to know the theory behind )

Regards

[ahmedtheking]

Just read the RSS feed, then append the ads and stuff and repurpose it! Nice and simple!