Click the piece that has an arrow pointing up.  It is on top of a vertical stack of upside down triangles
page7-257.png page2-129.png page3-46.png page7-203.png page6-34.png
page2-129.png page2-26.png page6-34.png page7-257.png page2-40.png page7-203.png page3-67.png page7-219.png page3-46.png page7-26.png
Keep that piece, and now click something similar.  The triangles on the bottom are both pointing down, and the arrow on top is pointing up