Lejos is an open source (free) firmware upgrade for the Lego Mindstorm RCX. "Legos Mindstorms" are a robotics kit sold by the Lego Corporation. The RCX is the brain of this kit (see the big yellow brick below). Normally, the RCX comes with firmware provided by Lego. However, using Lejos, the firmware can be overwritten with Java!

In my class, we build Mindstorm Robots after the AP test is over in early May. This provides a high interest, hands on activity for students that keeps them focused at the end of the school year. Below is a graphic showing our starter robot, known as the "Three Wheeler".