Quick Quiz

Here is a quiz for you, look at the picture and near the bottom of the gate you can see a plastic bottle filled with water tied to the gatepost. You come across this all over Spain (although it seems to me to be dying out a bit).  The question is why……

Apparently it is to stop dogs peeing (marking their territory) on the gatepost.  I have no idea if this works, if anyone has an idea on the theory please let me know.