According to tradition, maple syrup was discovered when a Native American woman, tired of going to a local watering hole for cooking water, gathered liquid flowing from a wound in a maple tree in late winter and early spring, found that it made her food sweet, and then boiled it down into a thick liquid. Early maple syrup production involved hollowing out a log, filling it with maple sap, and placing hot rocks in it until the water evaporated and syrup remained. Maple syrup is made by concentrating the slightly sweet sap of the sugar maple tree, which is collected in late winter and early spring when nights are below freezing and days are above freezing. Trees are tapped by drilling holes 7/16 of an inch in diameter and about 3 inches deep, inserting spiles or tubing taps, and collecting the dripping sap in buckets or carrying it downhill through tubing from multiple trees to a collection area. Sap flow depends heavily on weather, and cold or unseasonably warm conditions can halt production for days or even for a season. Sugar maple sap contains only about two to three percent sugar and 97-98 percent water, while maple syrup contains 67 percent sugar and 33 percent water. After sap is collected, water is removed in an evaporator with pans above a firebox, where the boiling sap becomes increasingly concentrated until syrup in the front pan reaches a temperature seven degrees above boiling and is drawn off. It is then finished in a propane-fired pan, filtered to remove impurities, reheated to kill potential bacteria, and bottled hot. Because of the labor and energy required, pure maple syrup is relatively expensive, and usually more than 30 gallons of sap are needed to make one gallon of syrup, though the amount can range from 30 to 90 gallons.