The habit of asking why is essential for students to develop sound mathematical reasoning. In one class, imagine a student wants to divide an 8 1/2 inchtimes11 inch sheet of paper into three columns of equal width. The student is ready to measure off lengths of 2 5/6 inches, but the teacher says, "Let me show you a carpenter's trick." He places a 12-inch ruler at an angle on the page so that the 0-inch and 12-inch marks on the ruler are on the left-and right-hand edges, respectively, and makes marks at the 4-and 8-inch points on the ruler. He then repeats the procedure, with the ruler farther down the page. Drawing lines through the 4-inch marks and the 8-inch marks divides the page neatly into three equal parts. The teacher then says, "Carpenters use this trick to divide boards into thirds (see fig. 7.33). My questions to you are, » Why does it work? Can you find similar procedures to divide a board into four, five, or any number of equal parts?