This was the cover for Hydro Review in March, 2003. We explored the theme of puzzles for many of these covers. This shows a toy puzzle that tasks the user to arrange tiles into a pre-determined pattern. I was driven mad by these as a child. This puzzle shows a common type of turbine called a Kaplan turbine, developed in the early twentieth century by Austrian engineer Viktor Kaplan. Many of these turbines are in use today, ranging in size from a few pounds to many tons.