The boys (12, 10 and 8) and I saw the second part of Peter Jackson's Hobbit series. My sons were mystified by what they considered a major plot flaw: what could Thorin Oakenshield possibly have used as a boat on the river of molten gold?
After all, it would have to have a lower specific gravity than gold (to float), a higher melting point (to avoid disintegration) and a low heat transfer rate (so Thorin wouldn't be grilled like a hotdog while riding it).
After much research, the eldest discovered the answer.
According to Peter Jackson, Tolkien's dwarves not only discovered mithril, they also discovered how to make aerogels.