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The code on the Arduino Bit tells it what to do. You can program this Bit to do different things by loading code onto it, and can share your code or upload others' directly - no IDE needed. To load code onto your Arduino Bit, connect it to your computer with the included micro USB cable. Once you've loaded the code, it will stay on the Arduino Bit, even if you disconnect it from the computer.
If you leave it connected, you can also use it to send messages back to your littleBits circuit and your computer. Just like all other Bits, the Arduino Bit needs to be connected to a power Bit in order to work. If you turn the power off, the Bit will stop working, but the code will still be on.