He knows if you've been naughty. Is it Santa? Or just your daughter's new teddy bear? Engineers at NeuroSky, Emotiv, and CyberLearning are betting on the next big thing in interactive play - toys that read your child's very brain waves and adapt play based on the results. In the offices of NeuroSky, the lightsaber on a Darth Vader toy lights up, controlled remotely by a user's brain waves. It is biofeedback writ large. While Vader is a crude demonstration of the types of toys that can use brain wave measurement as a play-feedback device, these start-ups envision games that require players to demonstrate mental focus in order to succeed in both virutal and real life play. Brilliant evolution of play, or the next step to a dystopian, Brave New World?