You Can't Do That On Television was a wacky TV show about teenagers and the squares that were in charge and starred Alanis Morissette, in her younger years.
Throughout the show, when someone said "I dont know", they would get slime dropped on them from above. It also had such memorable catch phrases as, when the cafeteria guy overhears the kids making fun of him, he always said "Duh, I hear that!".
Great late 80's / early 90's show on Nickelodeon.