I'm preparing to teach a demonstration lesson from Sunken Ships and Grid Patterns. Session One introduces the coordinate grid using a map of a town and states that the houses are at the intersections of the streets on the grids. This is an introduction to using coordinates to name locations. The next activity then involves finding distances between points on the same grid. Has anyone ever had students challenge this idea on the grounds that the streets are going though the houses? How have you dealt with this? Is this a flaw in the design of the investigation or am I missing something?