Reynolds' dilatancy is the observed tendency of a compacted granular material to dilate (expand in volume) as it is sheared. This occurs because the grains in a compacted state are interlocking and therefore do not have the freedom to move around one another. When stressed, a lever motion occurs between neighboring grains, which produces a bulk expansion of the material. On the other hand, when a granular material starts in a very loose state it may initially compact instead of dilating under shear. Reynolds' dilatancy is a common feature of the soils and sands studied by geotechnical engineers, and is a part of the broader topic of soil mechanics. It was first described scientifically by Osborne Reynolds (1842–1912) in 1885 and 1886.