[pbhjpbhj]

>build a bridge that could transport marbles from one side of a 12-inch square to another without anything touching the surface within the square

Could you roll the square in to a tube and roll the marbles along what is effectively the underside of the square. It doesn't touch the "[top] surface within the square. Might need 2 paperclips.

The obstacle is the bridge.

[pennig]

An interesting thought, but the square was just a masking tape outline that couldn't be messed with.