More than 300 years ago Sir Isaac Newton discovered gravity when an apple fell on his head. Today his theory, known as "the law of gravity," helps us better understand how things work such as water runoff on a home's exterior, and how and why leaks happen. It's just Newton's theory in action. When water gets into your home, understanding the principle will help you trace the leak. Home exteriors are designed with the fact that water runs downhill in mind. From the top of your roof peak down to the foundation, every element overlaps the one below it to let water run off the structure. When you've got a leak, running water has simply "defeated" the system. To find points of entry, reverse the law of gravity and (using a garden hose) test from the bottom up. Spray higher and higher until you find the source, giving special attention to windowsills, doorframes, loose shingles and damp stains. Remember: runoff is the key and gravity is the law. And that's the On The House tip for today.