Farewell C-3PO, we hardly knew you.

The world's most popular service robot (that is, not one that is in a factory) looks like an enormous ice hockey puck and vacuums your carpets.

The Roomba, made by iRobot, doesn't look anything like any of science fiction's robots, dating from their inception by Karel Čapek in his 1920 play R.U.R. (Rossum's Universal Robots), through Rosie the Robot in the 1960s Jetsons, and ending with any number of robots from Star Wars' C-3PO, through to Sonny in the 2004 movie I, Robot. We are supposed to be living in the age of the humanoid robot. So what went wrong?

Building human robots turns out to be a big challenge, said Professor Giorgio Metta of Genova's Istituto Italiano di Tecnologia. Humans are very, very good at being human. Robots aren't.

At the at the recent InnoRobo conference held in Lyon Mr. Metta was demonstrating iCub, a robot about the size of a small child. 'There is this general belief that robots will be very powerful. In fact electric motors are nothing like as powerful as human muscles,' he said. 'There are fundamental problems in materials, in the structure of the body.'

If it is so hard, and everyone agrees it is, why do it? Uwe Haass from the Cognition for Technical Systems at Munich's technical university, said the desire to build human-like robots is deeply rooted. 'It is the coronation of the creation. Many roboticists understand the human body as a wonderful creation of God or of evolution. Everything is so fantastic.'

Will Jackson, CEO of Engineered Arts Limited, a U.K.-based manufacturer of RoboThespian