There is an old episode of That '70s Show where Eric and his friends go to an adult film. While watching it Eric sees that all the couples do "something" and everyone seems to really like it. So Eric decides to try it on Donna without letting her know. Well, Donna doesn't like it. She freaks out. Later she says it was unpleasant. The show seems to drop hints at what Eric did to Donna but I can't figure it out. What sexual thing did Eric do to Donna that was unpleasant?