This paper presents an investigation by pre-service secondary school teachers in a geometry class of the relationship between the perpendicular distance from the eyeball to the wall (x) and the viewable vertical distance on the wall (y) using a view tube of constant length and diameter. In undertaking the investigation, students used tabular and graphical representations to determine the relationship. They also used a TI-84 calculator to investigate the relationship, and also modelled the scenario with the aid of "Geometer's Sketchpad" software. In this article, the author offers four reasons to use investigations and presents the first of three investigations posed to a group of preservice teachers in a geometry class for pre-service secondary school teachers in Texas, USA. (Contains 6 figures and 1 table.)