What's a filled bottle doing in the middle of the street?

Was back in Hanoi last week to conduct a workshop on network security. Got a chance to have a stroll along the streets around Sofitel Plaza hotel area with a Vietnamese colleague on the last day after the event. Along one of the street, I noted this bottle that was filled with a greenish-blue liquid and placed on top of a small brick, near to the middle of the street. Have never seen such a thing in other places. This surely raised my curiousity. What's the purpose of this bottle of liquid? Why is it placed in such a position? What's your thoughts? Write me a comment, make a guess.