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@@ -61,6 +61,7 @@ Some are in Jupyter (IPython) notebooks, some in `.py` files. You can view the f
 |[Symbolic Algebra, Simplification, and Differentiation](ipynb/Differentiation.ipynb)&nbsp; &nbsp; **[NB](http:/nbviewer.jupyter.org/github/norvig/pytudes/blob/master/ipynb/Differentiation.ipynb)**, **[DN](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmaster%2Fipynb%2FDifferentiation.ipynb)**<br>*A computer algebra system that  manipulates expressions, including symbolic differentiation.*|
 |[Economics Simulation](ipynb/Economics.ipynb)&nbsp; &nbsp; **[NB](http:/nbviewer.jupyter.org/github/norvig/pytudes/blob/master/ipynb/Economics.ipynb)**, **[DN](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmaster%2Fipynb%2FEconomics.ipynb)**<br>*A simulation of a simple economic game.*|
 |[How to Count Things](ipynb/How%20To%20Count%20Things.ipynb)&nbsp; &nbsp; **[NB](http:/nbviewer.jupyter.org/github/norvig/pytudes/blob/master/ipynb/How%20To%20Count%20Things.ipynb)**, **[DN](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmaster%2Fipynb%2FHow%2520To%2520Count%2520Things.ipynb)**<br>*Combinatorial math: how to count how many things there are, when there are a lot of them.*|
+|[Pairing Socks](ipynb/Socks.ipynb)&nbsp; &nbsp; **[NB](http:/nbviewer.jupyter.org/github/norvig/pytudes/blob/master/ipynb/Socks.ipynb)**, **[DN](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmaster%2Fipynb%2FSocks.ipynb)**<br>*What is the probability that you will be able to pair up socks as you randomly pull them out of the dryer?*|
 |[Euler's Sum of Powers Conjecture](ipynb/Euler's%20Conjecture.ipynb)&nbsp; &nbsp; **[NB](http:/nbviewer.jupyter.org/github/norvig/pytudes/blob/master/ipynb/Euler's%20Conjecture.ipynb)**, **[DN](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmaster%2Fipynb%2FEuler's%2520Conjecture.ipynb)**<br>*Solving a 200-year-old puzzle by finding integers that satisfy a<sup>5</sup> + b<sup>5</sup> + c<sup>5</sup> + d<sup>5</sup> = e<sup>5</sup>.*|
 
 |Computer Science Algorithms and Concepts|