One of the most beautiful and influential works of science. Its beauty lies in its logical development of geometry and other branches of mathematics. Comprises 13 books and the following web sites are worthy of a visit. A series of investigations using Geometer's Sketchpad. The website provides curriculum support for these topics in Maths A in Queensland, but has much wider applicability.
Teachers are free to download the resources for use within their classroom.
The website is a rich source of information, activities, resources, assessment and problems on these topics. Intuitively,there seems to be no such thing. A little research,I asked several strangers to point to a point C and got blank stares. I then asked them point to a line and they immediately pointed to lines in the parking lot, curbs, edge of buildings, roof lines,etc. In the mind,lines exist.