The growing parts of a tree are the cambium, root tips, and branch tips.
Growth rate depends upon factors such as moisture, light, nutrients, space, and insect infestation.
In some trees, especially softwoods, the rings are more distinct than in others. Obtain slices from a local arborist, forester, or from someone who cuts wood, making sure that you get ones with rings that can be counted easily without magnification. Distribute tree cookies. To see if everyone is getting the idea, ask what would happen if someone peeled off all the bark. Students can make an image of the rings of the tree cookie by placing paper over the cookie and rubbing crayon over the area.