Heat conduction mechanisms in nanofluids, fluids seeded with nanoparticles, have been extensively scrutinized in the past decades to explain some experimental observations of their enhanced thermal conductivity beyond the effective medium theory. Although many mechanisms such as Brownian motion, clustering, ballistic transport, and internanoparticle potential are speculated, experimental proof of any of the mechanisms has been difficult. Here, we investigate the mechanisms experimentally by thermal conductivity measurements and structural analysis for the same materials in both liquid and solid states. These studies strongly suggest that clustering holds the key to the thermal conductivity enhancement of nanofluids.