The final step is to string up the mobile. It is critical that you begin at the bottom. Find the center of gravity of the bottom piece by balancing it on your finger or a pencil. Poke a hole about 1/8" (3mm) from the top edge at the center of gravity. Next poke a hole through the end of the second smallest piece as shown in the image on the right. Take a short length of ordinary sewing thread and tie knots through each hole so that the pieces are about 2" (5cm) apart. Trim off any excess thread. Continue this process adding additional pieces. When you are checking for the center of gravity for each successive pieces, balance only the top piece on your finger. For example, after you have tied the second piece to the third piece, you need to find the center of gravity of the third piece - to do this, balance the third piece on your finger with the first and second pieces dangling. With this technique, the mobile will balance gracefully. To complete the mobile, tie a final string at the center of gravity of the topmost piece and hang it on a hook from the ceiling.