The Dog Riddle: https://zsmb.co/the-dog-riddle/

DogRiddle.kt

sealed class Animal {

    // Cats are simple. A cat is a cat.
    object Cat : Animal()

    sealed class Dog : Animal() {
        abstract val breed: String?

        class DogWithBreed(override val breed: String) : Dog()

        companion object : Dog() {
            override val breed: String?
                get() = null

            operator fun invoke(breed: String) = DogWithBreed(breed)
        }
    }
}