At the end of the day

Rest.

This building lark seems to be subject to a set of laws contrary to those of physics.  In some distant blip I articulated the first law which is that measurement changes over time.  So if you measured something today then it is unlikely tomorrow it will be the same.  I found this one out when taking my time sawing up wood to fit a particular, precisely measured space.  It didn't fit.

I think I discovered a second law.  If you mark a point on a wall to drill some hole for some fixing, then it will move when you take your eyes off it.  I discovered this when the holes for that to be fixed didn't line up.

I think there may be others.  I suspect pencils walk so that when you need it, it is never in the place you left it.