From the previous article in this series, "Fuelcells: Chip of the Future," [1] we learned something about how fuelcells differ from other energy conversion technologies and something about what a fuelcell does. But we don't yet know anything about why. To get to why, we'll now climb inside a fuelcell to learn how it works. We'll develop our understanding in layers, building upon the solid polymer electrolyte (SPE) configuration introduced in the last article. Although there are many different generic fuelcell types, at their most basic, all fuelcells share so many commonalities that we can almost say "seen one, seen 'em all." As a guide to fuelcell spelunking, we'll employ a schematic drawing of an SPE fuelcell and two linked equations. From these two aids-to-navigation we can see where the hydrogen and oxygen enter, how the electrons take off through an external circuit to deliver power and how, to balance the electrons, protons meander through the electrolyte. For some people the drawing will work best, for others the equations will make things tighter, but the best understanding will come from knitting together both to reinforce each other. Then, having seen where the various species travel, we'll burrow still deeper to learn what causes the electrical potential difference between the anode and cathode. This potential difference drives the whole process. Moreover, by understanding how the potential is created we will have a wonderful platform from which to discuss the four mechanisms that degrade fuelcell performance. In turn, understanding the four loss mechanisms makes it easy to understand the tradeoffs that govern all fuelcell designs. To me this step-by-step hierarchy of knowledge is one of the reasons learning about fuelcells is really fun-each bit of insight it fits nicely upon the preceding bit to build a solid scaffold of fuelcell understanding. With the experience of climbing about inside an SPE fuelcell we'll climb back out to talk about how fuelcells are classified, why, and how some types are more suited for some applications than others. Finally, I'll invite you to join me in a little speculation on what might come next in fuelcell development. The prospect of fuelcell spelunking might seem a bit daunting, because at times you'll need to jump back-and-forth between dialog, equations and diagrams. Still those willing to come with us on this voyage into fuelcell innards will, I think, enjoy having some appreciation of how fuelcells do their thing. Of course, we won't get all the answers. But you'll surely be able to ask intelligent questions of fuelcell engineers and scientists, and make intelligent comments when your broker suggests you invest.