Hi all, I am missing something utterly fundamental here. I have assembled a basic circuit on a breadboard, consisting of a 220 ohm resistor, an LED, and a 5V power source (an Arduino actually). Here is what I have. Assume the red wire is connected to a positive side of the voltage source and the black wire is connected to the negative side. In this configuration, the LED lights. However, if I rotate the LED and put the ground wire in what I believe to be in series with it, the LED does not light (had to rotate the board as well to make it clearer what had changed): I do not understand why the LED does not light in the above configuration. In my mind, the power comes in through the red wire, flows across the five-hole horizontal bus to one end of the resistor, flows through the resistor to the other end, flows across the five-hole horizontal bus to the anode of the LED, through the LED, out the cathode and to the black wire connected to ground. Whatever am I missing?