Wood: importance, structure, types and uses
Wood is a ready building material and the main material of carpentry. It is used to make buildings, parts of wagons and cars, plywood, furniture and sports gear. Its advantages: easy to work, light, strong, poor conductor of heat and electricity, not quickly destroyed by acids and alkalis, attractive, and it glues and finishes well. Its drawbacks: with moisture and temperature it dries and warps, swells and cracks, burns easily and cannot resist rot and insects. A tree consists of roots, trunk and branches; the wood-forming part of the trunk is made of the pith, the woody fibre and the bark. On a cross-section of the trunk annual rings can be seen, and their number shows the age of the tree.
Counting rings: the teacher gives a picture (or a ready smoothed cross-section sample) of a tree trunk. The group counts rings, finds the age and discusses wide vs narrow rings (good or poor years). Felling or sawing trees is never done by children.