This ad campaign started around 1975, and ran up until the early '80s. There would be 2 cups of soda on the counter (Coke and Pepsi), and the participant would take a sip from each and try to guess which was which (they were always wrong, btw). I used to think if I took the challenge I'd be able to guess which was which, but then they wouldn't have put me on tv, so I didn't pursue it..