Why is the 'when' expression in kotlin saying it's not exhaustive?

Example:

fun describe(x: Int): String {
    when (x) {
        1 -> return "One"
        2 -> return "Two"
    }
}

Solution:

fun describe(x: Int): String {
    return when (x) {
        1 -> "One"
        2 -> "Two"
        else -> "Unknown"  // Handling all other cases
    }
}

In Kotlin, when using the `when` expression as an expression (i.e., returning a value), you need to make sure it's exhaustive and handles all possible cases, including an `else` clause.