Plywood consists of several layers of wood glued together, and it offers an economical alternative to natural wood. It's normally available in four-by-eight foot sheets and is graded according to its use. If you're using it for a door, you'll want both facing sides to have smooth finishes. For panels that will have only one side exposed, like the underside of overhangs, you don't have to worry about, or pay extra for, a finish on the back. Structural uses, like the underfloor beneath hardwood or tile floors, don't require either side to be finished. Plywood can easily be nailed into position if you're driving nails through the face. If you plan to put it on hinges or use it in such a way that requires you to drive screws into the sides, you'll need to reinforce these areas. Drill a hole in the place where the screw will go, then glue a tight-fitting dowel into the hole. Screws can be driven securely into the dowels. The layers that make up plywood can separate if they get wet, so be sure to use a waterproofing sealant along the edges to repel moisture. Finally, always sand plywood, rather than using a plane. The facing layer is very thin, and could tear if it's subjected to too much stress.