D is for Dendrochronology

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" Chronology " is the study of the order in which things happen, and

"Dendro" means "related to trees", so

"Dendro chronology " is the use of trees to learn when things happened.

Each year as they grow, some kinds of trees add a thin layer of wood. This is how trees get bigger as they get older. When you look at a cross-section of a tree, these layers look like rings, which is why they're called "tree rings". If you count the tree rings, you can tell how old a tree is.