Val and Overrides in Scala

Question

As i understand vals are immutable in Scala. However in case of inheritance we can override a val defined in the base class. If the val member already got created when the base class was constructed then how can derived class override it? What am i missing?

class Animal ()
{
    val talk: String = "Ha Ha!"
}

class Dog extends Animal
{
    override val talk = "Woof Woof!"
}

class Cat extends Animal

@main def mainFn() = {
    var happyAnimal = new Animal()
    println(happyAnimal.talk)

    happyAnimal = new Dog()
    println(happyAnimal.talk)

    happyAnimal = new Cat()
    println(happyAnimal.talk)

}

Output

Ha Ha! Woof Woof! Ha Ha!