I've just had a conference with our resident and non-resident cats and they had some additional suggestions other than applying Miracle Whip or whatever it is called - too lazy to scroll up. They suggest that there should be a litter box for EACH cat in a different part of the dwelling unit, and that said boxes be cleaned out every day! Radical idea, indeed. They also recommended separate scratching posts, suspended baskets, that videotape with the birdies and squirrels, and a cat door with voice recognition features; they're only interesting in non-resident cats whom they invite in...