1944 - Lars Onsager (Nobel Prize in Chemistry, 1968) obtains exact solution for Ising model in two dimensions at critical point. The importance of this discovery is understood with universality - the same results obtained from this simple model apply to a wide variety of other systems in the same "universality class" (scientists always try to use the simplest model possible; in this case, the simplest model gives the exact same results as more complex ones!). Simple is a relative word, however; it was almost another decade before the rest of the scientific community was able to understand Onsager's work, and still more time before the solution was put into context of more "intuitive" approaches.