On this bridge, that crosses the Seine at the Louvre, people attach a padlock and then throw the key into the river as a symbol of their love.  There are several such bridges in Paris and others around the world.

 

I love this idea, but unfortunately all the added weight (it can be tons) can damage the bridges.

 

Eventually the locks have to be removed.  I wonder if this ever causes a spike in breakups???  Just curious.