One of the most astonishing properties when studying numerical series is that the sum is not commutative, that is the sum may change when the order of its elements is altered. In this note an example is given of such a series. A well-known mathematical proof is given and a MATLAB[C] program used for different rearrangements of the series converging to different numbers. It is noted that the code follows the mathematical idea of the proof of the property. Developing programs showing the main ideas in the proofs of the theorems may be a good tool in the understanding of such theorems and proofs. (Contains 4 tables.)