A special branch of communication involves the use of echoes to replace sight when visibility is poor.
Communication is so important that we often want to do it quickly and over long distances.
The body consists of many different organs each composed of enormous numbers of living cells.
Computers were first created as calculating machines for the manipulation of numbers.
Viewed from space, one of the remarkable features of our planet is that it has continents and oceans.
Volcanoes are among the most dramatic features of the Earth, and a sure sign that the planet is alive.
The continental crust is much, much older than the oldest surviving oceanic crust.
Water is an essential ingredient of the Earth, and without it, we would not be here.