The legs are retractable and are actuated by tiny servo motors inside the body. [Greg06] even included an ultrasonic distance sensor for the obstacle avoidance mechanism.

Remote-Controlled Car Transformed Into A Robot With Arduino. By Elizabeth ... Seeing as Arduino is good for controlling motors and sensors, Randy added a PING ultrasonic sensor to the front of the ...

The code includes two Arduino libraries: one for the servo and another for the ultrasonic sensors. The main code is a simple 14-line piece with a simple if-else statement you can readily modify.