When you call IO.readln the human on the other side can type whatever they want.

This means that, depending on the question you asked, you might need to interpret what they typed as something other than a String.

In its most basic form this will look like seeing if one String equals another String.

void main() {
    String color = IO.readln("What is your favorite color? ");
    if (color.equals("green")) {
        IO.println("Me too!");
    }
    else {
        IO.println("neat.")
    }
}