Line objects are used to define the edges of the level, as far as the Goo Balls and dynamic geometry are concerned. Lines are the simplest type of geometry object, however if you don't understand how they work, they can cause you all sorts of problems.

Lines have a position (anchor) and a normal direction (normal). Together these tell the game where the line is and what its rotation is. The normal dictates which way the line points and defines the front and a back. It's the front and back which are important, because Goo Balls and dynamic geometry objects can only ever be in front of a line.

In fact, they must be in front of all the lines in a level. You must always have the normal's pointing inwards towards the middle of your level. If they face outwards then Goo Balls are not allowed to be in your level.

If a ball or dynamic geometry is placed the wrong side of a line, the physics simulation will go berserk due to the illegal placement.

Lines have a tag attribute and a material, so they can affect Goo Balls, Structures and Dynamic Geometry in many different ways.

Unlike circles and rectangles, lines can't be placed into a compositegeom - not that this would make sense anyway.