diff --git a/flexo.js b/flexo.js
index 2e68fdf..5735537 100644
--- a/flexo.js
+++ b/flexo.js
@@ -849,13 +849,20 @@
 
   // Create a new urn to pick from. The argument is the array of items in the
   // urn; items can be added or removed later.
-  flexo.urn = function (items) {
-    return Object.create(Urn).init(items);
+  flexo.urn = function () {
+    return Urn.init.apply(Object.create(Urn), arguments);
   };
 
   var Urn = {
     init: function (items) {
-      this.items = Array.isArray(items) && items || [];
+      flexo.make_property(this, "items", function (items) {
+        this._remaining = [];
+        delete this._last_pick;
+        return items;
+      });
+      this.items = arguments.length === 1 && Array.isArray(items) ? items :
+        flexo.slice(arguments);
+      this.__id = flexo.random_id();
       return this;
     },
 
@@ -908,11 +915,6 @@
     }
   };
 
-  flexo.make_property(Urn, "items", function (items_) {
-    this._remaining = [];
-    delete this._last_pick;
-    return items_;
-  });
   flexo.make_readonly(Urn, "remaining", function () {
     return this._remaining.length;
   });
diff --git a/sketchbook/20140324-squares/index.html b/sketchbook/20140324-squares/index.html
index 11facf8..4a5a1b4 100644
--- a/sketchbook/20140324-squares/index.html
+++ b/sketchbook/20140324-squares/index.html
@@ -24,26 +24,33 @@
     <script>
 "use strict";
 
-var W = 51;
-var H = 37;
-var colors = flexo.urn(["#ff6a4d", "#0b486b", "#5eb26b", "#774f38", "#f8ca00",
-  "#9e0b46", "#a61416", "#222222", "#f8f9f0", "#4dbce9", "#06491d", "#f94179"]);
-document.body.style.backgroundImage = "linear-gradient(to bottom, %0, %1)"
-  .fmt(colors.pick(), colors.pick());
-
 var svg = document.querySelector("svg");
-svg.setAttribute("viewBox", "0 0 %0 %1".fmt(W * H, H * H));
+var numbers = flexo.urn(19, 23, 29, 31, 37, 41, 43, 47, 53, 59);
+var colors = flexo.urn("#ff6a4d", "#0b486b", "#5eb26b", "#774f38", "#f8ca00",
+  "#9e0b46", "#a61416", "#222222", "#f8f9f0", "#4dbce9", "#06491d", "#f94179");
 
-for (var y = 0; y < H; ++y) {
-  for (var x = 0; x < W; ++x) {
-    var sz = Math.max(1, flexo.random_number(y * 0.8, y * 1.25));
-    var x_ = x * H + (H - sz) / 2;
-    var y_ = y * H + (H - sz) / 2;
-    svg.appendChild(flexo.$rect({ x: x_, y: y_, width: sz, height: sz,
-      fill: colors.pick() }));
+function generate() {
+  var w = numbers.pick();
+  var h = numbers.pick();
+  document.body.style.backgroundImage = "linear-gradient(to bottom, %0, %1)"
+    .fmt(colors.pick(), colors.pick());
+  flexo.remove_children(svg);
+  svg.setAttribute("viewBox", "0 0 %0 %1".fmt(w * h, h * h));
+  for (var y = 0; y < h; ++y) {
+    for (var x = 0; x < w; ++x) {
+      var sz = Math.max(1, flexo.random_number(y * 0.8, y * 1.25));
+      var x_ = x * h + (h - sz) / 2;
+      var y_ = y * h + (h - sz) / 2;
+      svg.appendChild(flexo.$rect({ x: x_, y: y_, width: sz, height: sz,
+        fill: colors.pick() }));
+    }
   }
 }
 
+document.body.onclick = generate;
+generate();
+
+
     </script>
     <footer>
       Mon 24 Mar 2014, from the <a href="../index.html#mar2014">Flexo
diff --git a/sketchbook/index.html b/sketchbook/index.html
index ac9086a..3e3bdb9 100644
--- a/sketchbook/index.html
+++ b/sketchbook/index.html
@@ -281,7 +281,7 @@ <h1>Flexo Sketchbook</h1>
         </tr>
         <tr>
           <td>24<br>
-            <a href="20140324-squares/">Squares</a></td>
+            <a href="20140324-squares/" class="done">Squares</a></td>
           <td>25</td>
           <td>26</td>
           <td>27</td>