From http://www.news.cornell.edu/stories/May07/margot.mercury.html:
"Chefs have long used a simple trick to differentiate between a raw and hard-boiled egg. By spinning an egg and watching how it behaves when the spin is disrupted, it's easy to tell whether its interior is solid or liquid.
Applying a similar test to the planet Mercury, astronomers have found strong evidence that the planet closest to the sun has a fluid core. The research, led by Jean-Luc Margot, assistant professor of astronomy at Cornell, appears this week on the Web site of the journal Science.
Margot and collaborators conducted a series of observations over five years using a novel technique to detect tiny twists in Mercury's spin as it orbits the sun. The twists, called longitudinal librations, occur as the sun's gravity exerts alternating torques on the planet's slightly asymmetrical shape.
They found that the magnitude of the librations was double what would be expected for a completely solid body -- but explainable for an object whose core is molten and not forced to rotate along with its shell."
