That means you have not run the cell that defines the basic_sigmoid function or perhaps you ran it, but it threw an error because the code is incomplete or incorrect. Part of what we are learning in this “optional” tutorial assignment is how to work the Jupyter Notebooks. Please make sure not to skip the video about the notebooks in Week 2, which walks you through how they work. Also there’s the “Programming FAQ” reading item. There is also a topic on the DLS FAQ Thread about the ‘name not defined’ error in general.