A series of firsts on CBS' Big Brother (sex, exes, and one freaked-out chair chucker) have transformed the four-year-old reality show into summer's surprise hit. The thrice-weekly peekaboo show consistently wins its time slots in the advertiser-friendly 18 -- 49 demographic. Not bad for a house show that once seemed destined for foreclosurethanks to credibility killers like Justin, the knife wielder from season 2; and season 1's William Collins, a.k.a. Will Mega, the former member of the New Black Panther Party, whose leader, the late Khalid Abdul Muhammad, made anti-Semitic and antiwhite statements. The show has been picked up for three more years and BB3 will be released on DVD in stores Nov. 4. ''This has become a nice summer staple,'' says CBS CEO Les Moonves of BB. ''It's amazing how people get so involved.'' Exec producer Allison Grodner offers one explanation: ''It is the most naked cast we've ever had!''