We can learn a lot about things by studying how they move through the world and interact with the environment.
In the past, for example, it was possible to study the mobility of people within the United States by monitoring things such as the movement of banknotes. Today we can use something that is much more global and widely available than US cash.
Mobile phones have almost totally infiltrated human society, with the number estimated at more than 7 billion in 2014. Ownership of mobile phones continues to grow, even in some of