I am curious about this due to having a family member who is constantly whinging about the price of cat litter. These 2 cats go out whenever they like during the daytime, and at night the cat flap is locked so they can't go out, but regardless of whether they are outside or in, they always come inside to use the little tray. This seems a little mad to me, I always thought litter trays were mostly for kittens, or for cats which never go outside. Am I wrong?