I made my way through the Nerd Kits Guide. During the temperature sensor portion, I noticed that when I removed the thermometer, the temperature started fluctuating. Specifically what I am noticing is that the ADC is reporting a quickly fluctuating value between about 155 and 175 (out of 1024). Also, right when I plug in the battery, the number drops from closer to 400ish.

This surprised me ... a lot! I am using the sample code, and I have double checked all wiring. If I insert the temperature sensor, everything works fine. I'm just trying to understand where that voltage is coming from.

I looked at the datasheet. On page 246, there is a diagram of the ADC setup. I'm grasping at straws, but I could imagine that the input multiplexer is funky, or maybe breaking the circuit causes the multiplexer bits in ADMUX to choose the bandgap voltage of 1.1V (though I would expect closer to 225, and stability). Thoughts?

What you're seeing is capacitive and inductive noise. Without an input attached to override the noise, you're seeing a very sensitive ADC influenced by electrical activity in the MCU.

BM