Start on sum.

mathstuff.js

@@ -1,5 +1,7 @@
 (function( $ ) {
   $.fn.sum  = function() {  
-    return 0 
+    if (this.length == 0) return 0;
+    var value = this.val();
+    return isNaN(value) ? 0 : value;
   };
 })( jQuery );

mathstuff.tests.html

@@ -10,9 +10,33 @@
   <script>
   $(document).ready(function(){
 
-    test("sum returns zero when passed nothing", function() {
+    test("sum returns zero when given nothing", function() {
       var value = $('input[name=nothing]').sum();
-      equal( value, 0, "should return zero" );
+      equal(value, 0, "should return zero" );
+    });
+
+    test("sum returns 1 when given a single-item selector with value of ", function() {
+      $('input[name=one]').val(1);
+      var value = $('input[name=one]').sum();
+      equal(value, 1, "should return 1");
+    });
+
+    test("sum returns 2 when given a single-item selector with value of 2", function() {
+      $('input[name=one]').val(2);
+      var value = $('input[name=one]').sum();
+      equal(value, 2, "should return 2");
+    });
+
+    test("sum returns 0 when given a single-item selector with value of NaN", function() {
+      $('input[name=one]').val(NaN);
+      var value = $('input[name=one]').sum();
+      equal(value, 0, "should return 0");
+    });
+
+    test("sum returns 0 when given a single-item selector with non-numeric value", function() {
+      $('input[name=one]').val("abc");
+      var value = $('input[name=one]').sum();
+      equal(value, 0, "should return 0");
     });
 
   });
@@ -25,6 +49,10 @@ <h2 id="qunit-banner"></h2>
   <div id="qunit-testrunner-toolbar"></div>
   <h2 id="qunit-userAgent"></h2>
   <ol id="qunit-tests"></ol>
-  <div id="qunit-fixture">test markup, will be hidden</div>
+  <div id="qunit-fixture">
+    <form>
+      <input name="one" />
+    </form>
+  </div>
 </body>
 </html>