A Case-Insensitive Prompt Not Working Even With The Use of toLowerCase

I am learning JavaScript and have decided to do a basic Switch piece of code. I am trying to make my prompts' inputs case-insensitive using the .toLowerCase. However, that isn't working:

document.getElementById("button").onclick=function(){
var question = prompt("How do you do");
switch(question) {
case "Great".toLowerCase: 
   alert("Excellent!");
   break;
case "Good".toLowerCase: 
  var question_2 = prompt("Could it be better?");
   if(question_2 == "yes") {

       alert("Well lets hope tomorrow is better!")

   }
   else {

       alert("Phew! You got me for a moment!")

   }
   break;
   case "Bad".toLowerCase:
   var q_3=prompt("What happened!");
   if(q_3) {

     alert("I hope tomorrow is better!");

     }

     else {

    alert("I didn't quite get that..");

    }
    break;
    default: alert("I didn't understand...");
    }

    }

Every time I insert text, correct or incorrect, I will always get "I didn't understand", which I believes that any input is wrong. E.g, if I insert into the 'question' prompt "great" or "Great" or even "GREAT", I will always get "I didn't understand" as an alert. Any help?