diff --git a/stacks-queues/sort-stack.ipynb b/stacks-queues/sort-stack.ipynb
index 81c370d..0c83570 100644
--- a/stacks-queues/sort-stack.ipynb
+++ b/stacks-queues/sort-stack.ipynb
@@ -13,21 +13,26 @@
    "source": [
     "## Problem: Sort a stack.  You can use another stack as a buffer.\n",
     "\n",
-    "* [Clarifying Questions](#Clarifying-Questions)\n",
+    "* [Constraints and Assumptions](#Constraints-and-Assumptions)\n",
     "* [Test Cases](#Test-Cases)\n",
     "* [Algorithm](#Algorithm)\n",
-    "* [Code](#Code)"
+    "* [Code](#Code)\n",
+    "* [Unit Test](#Unit-Test)"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Clarifying Questions\n",
+    "## Constraints and Assumptions\n",
+    "\n",
+    "*Problem statements are often intentionally ambiguous.  Identifying constraints and stating assumptions can help to ensure you code the intended solution.*\n",
     "\n",
     "* When sorted, should the largest element be at the top or bottom?\n",
     "    * Top\n",
-    "* Can you have duplicate values, saw two 5s?\n",
+    "* Can you have duplicate values like 5, 5?\n",
+    "    * Yes\n",
+    "* Can we assume we already have a stack class that can be used for this problem?\n",
     "    * Yes"
    ]
   },
@@ -71,7 +76,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 1,
    "metadata": {
     "collapsed": true
    },
@@ -82,7 +87,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 2,
    "metadata": {
     "collapsed": false
    },
@@ -99,32 +104,68 @@
     "        return buff"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Unit Test\n",
+    "\n",
+    "*It is important to identify and run through general and edge cases from the [Test Cases](#Test-Cases) section by hand.  You generally will not be asked to write a unit test like what is shown below.*"
+   ]
+  },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 3,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Test: Empty stack\n",
+      "Test: One element stack\n",
+      "Test: Two or more element stack (general case)\n",
+      "Success: test_sort_stack\n"
+     ]
+    }
+   ],
    "source": [
     "from random import randint\n",
+    "from nose.tools import assert_equal\n",
     "\n",
-    "def test_sort(num_items, stack=None):\n",
-    "    if stack is None:\n",
+    "class Test(object):\n",
+    "    def get_sorted_stack(self, numbers):\n",
     "        stack = MyStack()\n",
-    "        for _ in xrange(0, num_items):\n",
-    "            stack.push(randint(0, num_items))\n",
-    "    sorted_stack = stack.sort()\n",
-    "    for _ in xrange(0, num_items):\n",
-    "        print(sorted_stack.pop())\n",
-    "    return sorted_stack\n",
+    "        for x in numbers:\n",
+    "            stack.push(x)\n",
+    "        sorted_stack = stack.sort()\n",
+    "        return sorted_stack\n",
     "\n",
-    "print('Empty stack')\n",
-    "test_sort(0)\n",
-    "print('One element stack')\n",
-    "test_sort(1)\n",
-    "print('Two or more element stack (general case)')\n",
-    "sorted_stack = test_sort(5)"
+    "    def test_sort_stack(self):\n",
+    "        print('Test: Empty stack')\n",
+    "        sorted_stack = self.get_sorted_stack([])\n",
+    "        assert_equal(sorted_stack.pop(), None)\n",
+    "\n",
+    "        print('Test: One element stack')\n",
+    "        sorted_stack = self.get_sorted_stack([1])\n",
+    "        assert_equal(sorted_stack.pop(), 1)\n",
+    "\n",
+    "        print('Test: Two or more element stack (general case)')\n",
+    "        num_items = 10\n",
+    "        numbers = [randint(0, 10) for x in xrange(num_items)]\n",
+    "        sorted_stack = self.get_sorted_stack(numbers)\n",
+    "        sorted_numbers = []\n",
+    "        for _ in xrange(num_items):\n",
+    "            sorted_numbers.append(sorted_stack.pop())\n",
+    "        assert_equal(sorted_numbers, sorted(numbers, reverse=True))\n",
+    "        \n",
+    "        print('Success: test_sort_stack')\n",
+    "\n",
+    "if __name__ == '__main__':\n",
+    "    test = Test()\n",
+    "    test.test_sort_stack()"
    ]
   }
  ],