Math Fun
https://www.nctm.org/Publications/mathematics-teacher/2010/Vol103/Issue6/Activities-for-Students_-Moving-a-Wall_-Using-Geometry-to-Measure-an-Imperceptible-Distance/
https://www.nctm.org/Publications/mathematics-teacher/2010/Vol103/Issue6/Activities-for-Students_-Moving-a-Wall_-Using-Geometry-to-Measure-an-Imperceptible-Distance/