A tall tree can transport a hundred gallons of water a day from its roots deep underground to the treetop. Is this movement propelled by pulling the water from above or pushing it from below? The pull mechanism has long been favored by most scientists. First proposed in the late 1800s, the theory relies on a property of water not commonly associated with fluids: its tensile strength. Instead of making a clean break, water evaporating from treetops tugs on the remaining water molecules, with that tug extending from molecule to molecule all the way down to the roots. The tree itself does not actually push or pull; all the energy for lifting water comes from the sun's evaporative power.