Our laboratory explores a fundamental question: what is life? We investigate this by examining the physical distinctions between living and dead cells. Our focus is cytoplasm, a crowded protein environment with remarkable properties. Though it contains protein concentrations that would gel or glassify in vitro, living cytoplasm maintains fluidity, enabling protein interactions, enzymatic reactions, and organelle transport to proceed as efficiently or even faster than in dilute solutions. When cells die, undergo metabolic inhibition, or enter dormancy, cytoplasmic fluidity decreases and gelation occurs. This suggests the existence of metabolism-coupled fluidization mechanisms, though their nature and effects on protein dynamics remain unclear.