Instructions: Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Input: Recently, I was cruising on the Danube river from Bratislava to Vienna and we were encountering these small green rocket-shaped objects floating in the water roughly after each kilometer. They were not staying still but also sailing in the same direction as our boat but slower.

What are these UFOs (Unidentified Floating Objects)?

Output:
What are these small green rocket-shaped objects floating in the Danube?