How can a class be declared in Kotlin so that it can be extended in inheriting classes?

In Kotlin, all classes are final by default, meaning they cannot be inherited. To allow inheritance, a class must be declared with the open modifier.

Example:

open class Animal {
    open fun sound() {
        println("Some sound")
    }
}

class Dog : Animal() {
    override fun sound() {
        println("Bark")
    }
}

Here, Animal is declared as open, which allows the Dog class to inherit from it and override its methods.