In situation two, however, I have the same two blocks at the same two temperatures and a solid bridge connecting the two blocks. Okay? In this situation, conduction will occur because of this metal bridge, and energy will be transferred. Energy flow will occur in this direction. And in this situation, there is no physical bridge that connects the two situations. Yes, there are air molecules flying around in this space here, but there is no solid physical thing that's connecting the two systems, and therefore, conduction will not occur.