Planning to cut a big hole in your wall to install a light or medicine chest? What's back there anyway? Pipes? Wiring? Gas lines? To keep from making a big mistake, it pays to do some studying and checking before you start. Otherwise you could flood the house and black out the neighborhood. Here's how to avoid these pitfalls: Walls are full of things that run your home. Those with sinks and washing machines usually contain plumbing. Electric wiring often runs up and down studs -- but not always. If you see vents on a wall, chances are good there's ductwork behind it. Study what's there; tap and listen for dense spots. Use a stud-finder and drill a small peephole. Common sense leads to good decisions -- just as if you had x-ray vision. And that's the On The House tip for today.