Back when I was a kid, a local expression used sometimes was "stupid simple".  That meant that something was beyond just simple.  It was so simple that even somebody real dumb could have come up with it, if they'd put their mind to it. 

I was checking string length between the bridge and nut on the violins before morning tune-up when I realized that the last time I had the bridge off the electric, I hadn't put graphite in the notches for the string.  I hadn't thought about it, since I hadn't actually done any work on the bridge that time.  But graphite is an ultra-fine powder.  It doesn't stick well to anything, which is why it makes a good lubricant for things like string grooves.  A teeny bit gets lost every time we play, and so it isn't a bad idea to put some on any time you have the strings out of the groove for any reason.  It is just pencil "lead", not like it is expensive.

Putting in a bit of graphite is also not a bad thing to try if your strings act a little oddly when tuning up.  Like if you felt the peg move, but the tuning didn't seem to change much or at all, and then later the string is out of tune in the other direction?  That can be the string "sticking" a bit at either the bridge or nut.  A little graphite will often fix it right up.

Usually I'd just use the point of a pencil, But if I slip even a little, then I get a little scribble near the notch, and i have to take an eraser to it and that is a minor bother. 

So I made a tool for it, similar to one I made for actually cutting the notches.

I took a popsicle stick, sanded one side to make sure it was reasonable straight and scotch-taped the .7mm #2 "lead" from a cheap mechanical pencil to it.

Like I said, "stupid simple".  But it worked well.  I got just a tiny dot of graphite right where the string rides in the notch, and it was actually easier than doing it with a pencil.  So into my musical instrument tool kit it goes!