In this activity, learners calculate the number of milliliters of water a nearby tree transpires per day. First, teams of learners go outdoors to collect five leaves from a mulberry or cottonwood tree. Next, learners calculate the area of each leaf and the total area of the five leaves together. Then, learners submerge the leaves in a beaker half full of spring water with a small layer of mineral oil on top. After 24 hours, learners estimate how many milliliters of water "transpired" (dropped) and divide this number by the total area of the five leaves to calculate the amount of water transpired in milliliters per square centimeter of leaf in 24 hours. Then, they make a few more calculations to obtain the number of milliliters transpired by the whole tree in one day. This lesson guide includes other calculations that learners can compute including the number of liters of water transpired per day and the yearly water loss.