title: Racket: using big bang and on-click
body: <p>I've somehow been able to draw a small basic image in racket (a house). Now, what I'm stuck with is, using big-bang to draw the image -- and then being able to click the roof of the house and have it change to another color. This is what I have so far: </p>
  
  <pre><code>#lang racket
  (require 2htdp/image)
  (require 2htdp/universe)
  
  
  
  (text "---Small Shack---" 18 "brown")
  
  (define door (rectangle 30 50 "solid" "white"))
  
  (define shack (above (triangle 100 "solid" "red")
                       (rectangle 120 80 "solid" "grey")))
  
  (define doorhandle (overlay/align "right" "center" (circle 5 "solid" "blue") door))
  
  (define house-view (overlay/align "center" "bottom" doorhandle shack))
  
  house-view
  </code></pre>
  
  <p>I've tried a million different things to get it to work with <code>big-bang</code> but nothing worked. Any help getting <code>big-bang</code> and it changing color of the roof when clicked? </p>