Scientists have made negative-temperature systems before, using the spins of atomic nuclei. Picture a line of atoms, each with a spin that can point up or down. In the lowest possible energy state, all spins point down. Add energy to the system and the spins will start to flip up — reaching maximum entropy, or disorder, when half the spins are up. Adding more energy after that will shift the system into negative temperature, whose high-energy states are the only way to accommodate the extra energy.