I have a robotic species on a distant planet (not necessarily similar to Earth, but it is indeed rich in metals and other materials necessary for making robots). They are born in a factory (duh!), made by "hand" from other robot(s).
They are programmed (by an ancient civilization that perished a long time ago) to preserve themselves as a species. Each individual is born with the identical program, which is slowly altered to suit the current need of the society. Note that there is no centralized program that tells them what to do; they are all autonomous, but prioritize their species as a whole.
The interesting part is that they are manufactured as a small robot, analogous to a 5 years old kid, and grow up to adult human size. This is always the case for all robots, although the actual process differs for each robot: some may grow slowly, some may have a longer arm, but all of them will always grow into a humanoid shape - always bipedal, with two arms, and a head, and a body.
How does this process of "growing up" work in robots?
The answer should address two things:
- Their limbs physically slowly grow larger over time.
- All robots only reach adult human size.
I'm not asking why (they are programmed to do so, just like our DNA influences our appearances), but how would this process actually work? They don't have cells like we do. They are composed of metals and wires just like our robots - but in a far more sophisticated way. So, no nanobot - they just don't reach that technological level since their idea of a robot is always macroscopic (based on their creator program), and all robots on the planet are always humanoid.
And yes, they are sentient. And no, they don't obey Asimov's Law. But don't worry, they don't know planet Earth. Yet.