In this blog post, the author explains how to build a digital twin of a toy car using an Android phone, MQTT broker, and Hasura GraphQL engine. A digital twin is a virtual representation of a real-world physical system or product that serves as its digital counterpart for practical purposes such as system simulation, integration, testing, monitoring, and maintenance. The author outlines the steps to create a digital twin, including setting up the toy car with sensors, streaming sensor data to an MQTT broker, receiving data from the MQTT broker and sending it as a GraphQL mutation to Hasura GraphQL engine, consuming live data on a ReactJS application with Hasura GraphQL subscriptions, and rendering real-time gyroscope sensor data as a 3D car model using ThreeJS. The author also discusses various use cases for digital twins, such as connected cars, remote monitoring, and remote debugging.