How to build a simple neural network in 9 lines of Python code

As part of my quest to learn about AI, I set myself the goal of building a simple neural network in Python. To ensure I truly understand it, I had to build it from scratch without using a neural network library. Thanks to an excellent blog post by Andrew Trask I achieved my goal. In this blog post, I'll explain how I did it, so you can build your own. I'll also provide a longer, but more beautiful version of the source code.