Is it true that there are more stars in the universe than there are grains of sand on Earth? How could one calculate such figures?
Andy Johnston, Kalaheo, Hawaii
It's probably not true, yet we can't answer with certainty. But this question is a wonderful example of what we sometimes call a Fermi problem, after physicist Enrico Fermi, who asked the "big" questions. The key to solving them is that you don't need to know the answer to any decimal places at all. You can be wrong by a factor of 10 and that's close enough.