Have any idea what this is?
A stud finder.
A magnet hanging from a string will find the drywall screws. How simple is that?! I had my $40 laser-level-stud-finder going along our walls and couldn’t find anything. Enter my handyman’s little device and presto!
Krisjan’s tip is to start on either side of an outlet or lightswitch, as these boxes are always attached to a stud. Generally speaking, you can find stud centers 16″ apart, but in our cheapo house it turned out to be 24″ and sometimes 32″ between studs!
I love cheap solutions like this, especially when they are so much more accurate than some techno gadget. My four-year old thinks it is “so magic!”