The key point is that the Ising model has a spin-spin interaction, making it energetically favourable for two adjacent spins to be aligned. On the other hand, thermal fluctuations typically introduce a randomness into the alignment of spins. At some critical temperature, T c , spontaneous magnetization is said to occur. This means that below T c the spin-spin interaction will begin to dominate, and there is some net alignment of spins in one of the two directions.