It grows just inside the bark of a tree. The thickness and appearance of this new wood depends on that year's weather and other conditions. So these extra layers of wood are like clues to the riddle of Earth's weather from long ago. If you cut down a tree, you'd see a series of tree rings nestled inside each other. Each ring is made from the wood that grows every year.
Scientists call the study of tree rings dendrochronology.
It's a word whose ancient roots mean tree and time. Scientists don't have to cut down trees to study tree rings. The study of tree rings can tell us about droughts and floods. Tree ring studies are one reason some scientists believe that Earth has been warming up.