Repeat switch statement until it has a valid answer

opts = prompt("Do you want to TALK or LOOK?").toUpperCase();

switch(opts) {
  case "TALK":
    mes = "Is anyone in charge here?";
    speech = "Our leader is in the big building.";
    talkNot();
    break;
  case "LOOK":
    window.alert("The buildings are quite simple, and the doorways are much shorter than you. You notice, however, that there is a very tall, thin building at the end of the street.");
    break;
  default:
    window.alert("That's not an option.");
}