Final answer:
Wood swells primarily in width when exposed to moisture.
Step-by-step explanation:
When wood is exposed to moisture, it absorbs the water, causing it to swell. This swelling occurs primarily in the width of the wood, rather than the length. As the water molecules fill the empty spaces in the wood's structure, the wood fibers expand outwards, increasing the overall width of the piece of wood. It's important to note that the length of the wood may also experience minimal expansion, but the width is more significantly affected.