Answer: Seed Type

Most people think of hard and soft wood types in terms of building and furniture material. Oak is a hardwood, pine is a softwood, and so on. Given that common context, it would be easy to assume that the terminology, hard versus soft, has everything to do with the physical structure and density of the wood.

Curiously, however, it has absolutely nothing to do with the actual wood. In fact, Balsa wood, the extremely light and low-density wood favored by children for making gliders, model rocket fins, and other uses in hobby crafts, is a hardwood.

The distinction between the two wood classifications is entirely dependent on the type of seed it produces. Hardwood trees are angiosperms, plants that produce seeds with a protective layer. Oak, then, is not a hardwood because it produces a sturdy and dense wood, but because it reproduces via acorns–a tree seed wrapped in a hard protective shell. Pine, to use a common softwood as an example, is a gymnosperm and reproduces by dropping unprotected seeds onto the ground (the seeds might be temporarily bound up inside a pine cone, but once the pine cone matures, the seeds are released to blow, wash, or otherwise be carried around by mother nature).