A QR (quick-response) code is a two-dimensional scannable code, similar in function to a traditional bar code that one might find on a product at the supermarket. The main difference between the two is that, while a traditional bar code can hold a maximum of only 20 digits, a QR code can hold up to 7,089 characters, so it can contain much more information. QR codes are versatile tools to extend learning inside and outside the classroom. Educators can use them to give students quick access to additional content, from the practical--such as links to assignments and websites--to the engaging--such as audio and video clips or clues for games. And, as their name suggests, QR codes save precious class time by speeding up activities. They are a fast, easy, free, and fun way to spice up one's lesson plans and school communication by linking the brick-and-mortar world to the digital domain. This article presents a primer and some inspiring ideas to get started using QR codes in the classroom. The authors discuss how to make them, how to read them, and how to use them inside the classroom and out.