A bridge that is permanently rebuilt by producing new paving stone. Can be combined with traps.
The background is a simple timed circuit that allows the piston to move forward and backward. The lava and water constantly produce cobblestone and push it forward. One application would be a booby trap at the beginning of the bridge, after which the bridge regenerates itself. But this is only one possible application. Another application would be the absolutely safe crossing of gorges, and, and, and.