Lagrange's Four Square Theorem states that every natural number can be expressed as the sum of four square numbers. For example, the number 7 can be represented as 4 + 1 + 1 + 1 (which is 2^2 + 1^2 + 1^2 + 1^2). This theorem was proven by the mathematician Joseph-Louis Lagrange in 1770 and is a significant result in number theory. The theorem implies that no matter how large or complex a number is, it can always be broken down into four squares. This result has important implications in various areas of mathematics, including algebra and combinatorics, and has inspired further research into the properties of numbers and their representations.