Victoria Keel, age 9, of Berea, Ohio, for her question:

What causes knots in wood?

The kind of wood that is marked with knots is cut from evergreen trees. Pine trees and Christmas trees and other cone bearing trees are stuffed with gummy resins. They also tend to grow thick, straight trunks with lots of smallish side branches. When they are small, many of them look like Christmas trees with pointed tops and wider branches right down to the ground. When they get to be tall trees, the lower branches have disappeared. But the tree trunk grows taller only at the top. So it does not lift up the lower branches as it grows. What happens explains how the knots are caused in the wood.

When a pine tree reaches the teenage stage of life, it begins to drop its lower branches one by one. As a branch falls, the gummy resin in the woody cells seals up the wound with a hard, darker colored scar. The trunk grows thicker by adding new rings of new wood around its outer edge. As bane goes on, the hard old scars from the fallen lower branches are buried inside the trunk. When the trunk is sliced into boards of lumber, the scars are revealed as decorative knots in the wood.