Wood is hygroscopic, which means it absorbs water. Therefore when surrounding humidity changes either through climate or the use of heating or air conditioning, solid wood can swell and shrink. This results in gaps opening between planks.
Engineered wood is made up of layers. The top layer/wear layer/ lamella is usually a hardwood and the base layers are softwood or multi layered plywood. All layers are bonded together so that the grain of each one runs perpendicularly of the layer on the other side making it impossible to swell or shrink.
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