CO₂ has T_c = 31.1°C. Can it be liquefied at 40°C by compression? Step 1: 40°C > T_c (31.1°C). Step 2: Above critical temperature, molecules move too fast for attractions to condense them. Step 3: No amount of pressure liquefies it — it becomes a supercritical fluid. Step 4: Cool below 31.1°C first, then compress → liquid CO₂. This is why CO₂ fire extinguishers work at room temperature (liquid inside!) but He needs cryogenics.