So TheLimey and I were discussing what kinds of systems are already functioning in our mini-offspring, and how tiny those little systems were. He seemed overcome with the realization that there were functioning mechanisms (i.e. heart, digestive tract, hearing, etc.) that could be that small, and suggested that this meant that eventually we (humans, that is) could create robots that small. (To play with!)
"But why couldn't we do that now?" I asked.
"Because you can't make functioning mechanisms that small," he replied.
"Well, yes, I can, as a matter of fact," I retorted. (I really wish I had thought to say "Au contraire, mon frere.")
We giggled at that for about two minutes, during which time I was subjected to the characterization of being a smartypants, though perhaps not in those exact words. (Personally, I like to think of myself as a skilled manufacturer of extremely small systems.)
Subsequently, I pointed out that there were already very tiny functioning biological mechanisms, such as those in insects and those micro-fish that were recently discovered. I suspect it's just that growing one of our own design (you know what I mean) makes it seem more real and salient.
So clearly, it isn't lack of biological examples that keeps us from mini-robots, but the state of our technology.