Could we use our body heat to generate electricity?

Andrew - Yes, that is quite possible. Humans are 30-35 degrees C versus the ambient temperature of maybe 20-25 degrees C. A small temperature difference is sufficient to generate a small amount of power and electronics are getting more and more efficient; we are now down at microwatt level for various things. So yes, you could quite feasibly have human powered electronics using TEGs as the mechanisms for that power.