Because a straight line is the shortest path it can follow between two points. However a locally straight line may not be a globally straight line. This is how a gravitational field bends the path of...
Depends. Probably not because nothing is perfect! :) i have to disagree - the sides of crystals are straight ~~~ *Sigh* All lines are straight. A man-made one may not be, but by the correct...
No. A circle has no straight lines. You can approximate circles with very small straight lines (that's how a computer draws circles) but mathematically, no.