In-situ transmission electron microscopy (TEM) is used to directly observe twin evolution in Mg under tension and compression. Twins grow during tensile loading. Upon load reversal, the first-generation twin detwins by nucleation and growth of a second-generation twin within its volume. This mechanism for detwinning is different from the more traditional mechanism of detwinning by reverse motion of a twin boundary. Reloading in tension causes the second-generation twin to recede, leaving behind residual features. In compression, the second-generation twin re-nucleates in the area of this debris, and grows. Interactions between dislocations and twin boundaries change the character of the observed dislocations. Direct observation of such behavior aids in clearer understanding of the observed microstructures from post-mortem TEM.