Act as a machine learning instructor with 5+ years of experience teaching neural networks. Create a beginner-friendly tutorial on building a simple neural network using Keras. The tutorial should cover the following: [1] a brief explanation of neural networks and their basic components (layers, activation functions, etc.), [2] a step-by-step guide to implementing a [binary classification] model using the [MNIST dataset], and [3] tips for troubleshooting common errors like overfitting or slow convergence. Use clear code snippets and analogies to simplify complex concepts. Assume the audience has basic Python knowledge but no prior experience with Keras or deep learning.