Publisher Summary To be correct, the extraction of the constitutive elements contained in the delimited increment must respect the “rebounding rule” or “rule of the centre of gravity”. In other words, the extracted increment is correct when, and only when, it is made of those particles (fragments, ions, molecules) whose centre of gravity falls within the boundaries of the delimited increment. This is true whether the delimitation has been correctly carried out or not. Delimitation and extraction are two strictly independent operations. The problem posed by the extraction is one of mechanical interaction between two sets of objects moving in relation to one another. (1) On one hand, solid fragments that form either a more or less compact stream that falls freely by gravity or a stationary pile and on the other, parts of either the sampling cutter (edges, walls, and bottom) as the latter crosses the stream to collect an increment, or the sampling tool, for instance a drill recovering cores from a stationary object, such as a pile or an unconsolidated orebody.