Trees have a vascular system just like us - xylem and phloem instead of veins and arteries. Xylem makes the dark rings that we can count to see how old a tree is.

We used a formula for estimating the age of a cottonwood tree:

First we found the circumference of the tree. We did this two ways: with kids and with a tape reel. It was seven kids or 198 inches. We decided to use inches.

We used a calculator to divide the circumference (198 inches) by 3.14 to find the diameter of 63 inches.

Our formula told us that cottonwoods increase 2 inches in diameter each year. We multiplied 63 by 2 and got 126. Our tree is approximately 126 years old.