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.

Instructions:

The only thing that is not self-explanatory is perhaps the circuit. It is a simple clock generator that is part of the Redstone 1x1 and can be recreated from the picture. There are no underground circuits.