“When neighboring spins agree, i and i + 1 are either both up or both down. We can intuit that since they agree, then the interaction energy between them will be less. On the other hand, when neighboring spins disagree such that, either spin S_i is up and its neighbor's spin S_i + 1 is down, or S_i is down and its neighbir S_i + 1 is up, then there is more interaction energy caused by the tension between the disagreeing spins. One can imagine two people in a discussion. If they agree, there will be less to discuss, less interaction. If they disagree, they will interact more, trying to shift the other's viewpoint.”