Cynthia Woods, age 11, of Burnaby, British Columbia, Canada, for her question:

How do knots form in wood?

Those handsome knotty woods are created by the conifer trees. They do it with gummy resins, made to seal their wounds and protect themselves from the winter weather. Like all trees, they grow taller by adding new material at the top. The trunks grow thicker by adding rings of new growth around the edges, but they do not stretch higher. The conifers create tacky resins with all their new woody cells.

A young pine is shaped like a Christmas tree, with a pointed top that slopes out to the widest branches brushing the ground. As the top stretches taller, the lower branches fall off one by one and others grow higher on the trunk. Each lost limb leaves a wound. Tacky resins ooze into the area and seal it like a sturdy scab. In time the gob of resin gets harder and darker and year by year the tree covers it over with rings of new wood. When the tree is grown, the scars from its lost branches are buried deep inside the trunk. They are the handsome knots we see when the wood is cut into slices of lumber.