SL Parallel & Perpendicular Lines
PARALLEL: train tracks that MUST never cross, but there are many other situations in which "two straight lines will never cross" e.g. the path of two ships on the sea/ocean, two lanes full of cars (though not always straight!), the strings on an instrument etc. PERPENDICULAR: if gravity didn't act at 90° to the earth's surface, maybe we would be less interested in the properties of 90° angles? Yet, right-angles...