The world's smallest programmable robot is the size of a grain of salt.
Why it is true
The robot measures only 200 by 300 micrometres wide and 50 micrometres thick, smaller than a grain of salt. It can swim through fluids, measure temperature and communicate data with a little dance, powered by solar cells generating about 100 nanowatts.
Source: MIT
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.