At the risk of sounding completely ignorant, how hard is it to make a thermostat? I opened one up and it seemed to be a bit of mercury that completed a circuit.

At the risk of sounding completely ignorant, how hard is it to make a thermostat? I opened one up and it seemed to be a bit of mercury that completed a circuit. Is slapping a UI and a "learning" system on that big of a deal? I can see how it's a useful product and might seem really cool, but it seems sort of simple to make. Sure, making physical products isn't as easy as software and all, but still, what am I missing?