In general, the robot needs to have the right expressive abilities to be engaging and helpful, but what those are depends on the robot's task and context. We've used the humanoid robot Bandit, which has arms, to show stroke patients how to do rehabilitation exercises, elderly users how to do chair aerobics and children with autism how to imitate movements. In other cases, we needed the robot to be encouraging and fun, but smaller and inherently safer, so we used the owl-like robot, Kiwi, in the homes of children with autism.