Applied practical geometry (safe for home use)

“An interesting application of the theorem relating to similar triangles is this: Extend your arm and point to a distant object, closing your left eye and sighting across your finger tip with your right eye.  Now keep the finger in the same position and sight with your left eye.  The finger will then seem to be pointing to an object some distance to the right of the one at which you were pointing.  If you can estimate the distance between these two objects, which can often be done with a fair degree of accuracy when there are houses intervening, then you will be able to tell approximately your distance from the objects, for it will be ten times the estimated distance between them.” – David Eugene Smith, The Teaching of Geometry (1911)