How to find and track a leak with the napkin trick

It is raining and a great opportunity to check for leaks and use the new trick from the book. The napkin trick is where you use a napkin to track a waterleak by placing it around where you think there is a leak, when there is a leak it will show on the napkin as it gets damaged in the surface when it get wet. When it is dry again the damage in the paper is still there and you can then see where the leak is.