In this codelab, you will learn how to build a simple "teachable machine", a custom image classifier that you will train on the fly in the browser using TensorFlow.js, a powerful and flexible machine learning library for Javascript. You will first load and run a popular pre-trained model called MobileNet for image classification in the browser. You will then use a technique called "transfer learning", which bootstraps our training with the pre-trained MobileNet model and customizes it to train for your application.