Installing shelves? Hanging a mirror? You'll need more than drywall to hang them on. You'll need the solid backing of a "stud" -- the vertical 2x4s behind finished walls. Here's six ways to locate studs -- ranging from high-tech to low-tech to no-tech to what the heck. Most reliable is the high-tech electronic stud-finder that detects changes in density. Pass it over a wall, and lights and tones tell when you've found a stud. Some signal the location of pipes, cable and wires, too. Low-tech magnetic finders locate screws and nails and also find wiring, pipes and air ducts that may be nowhere near a stud. No-tech methods include tapping and listening -- or noting where switches and outlets are installed, and eyeball techniques like holding a light close to a wall and looking for nail heads and seams. The "what-the-heck" method involves drilling a hole and being lucky enough to hit a stud. If you don't, insert a coat hanger and fish around inside for a stud. And that's the On The House tip for today.