The unmistakable sound of a knuckle cracking is caused by a gas cavity forming between finger joints, according to a new study.

The cavity, or bubble, forms in the synovial fluid, the lubricant between joints. The discovery was reported in the journal PLOS One.

One of the paper's authors, Jerome Fryer, a chiropractor in British Columbia, lay inside a cine MRI machine, which stitches together video from a series of rapid scans. Researchers at the University of Alberta cracked each of his knuckles by pulling on a cable attached to his fingers.

"You'll see the black cavity that occurs just as the cracking occurs" on the video, said Dr. Greg Kawchuk, a professor of rehabilitation medicine at the university and the lead author.

In 1947, researchers in England first theorized that the characteristic popping sound occurs when a gas cavity forms. Then, a group of researchers in the 1970s suggested it was the collapse of the cavity, not the formation, that caused the cracking.

Kawchuk added that knuckle cracking did not cause arthritis. "Those are just tales," he said.

On the contrary, he said, understanding exactly why knuckle-cracking doesn't seem to harm the joints could help researchers develop better therapeutic materials for patients with osteoarthritis and other degenerative joint diseases.

Pollen-deprived Bees are poor dancers

Worker bees without access to adequate pollen early in life turn out to be poor foragers, and dancers, as adults.

The bees' so-called waggle dance, a figure-eight movement, is used to tell other members of the colony how far and in what direction to fly to find flowers. If the pollen-deprived bees went out to forage, they often did not return, said Heather Mattila, a biologist at Wellesley College.

Mattila and Hailey Scofield, an undergraduate student, raised one group of bees with limited access to pollen and another with adequate pollen. They combined the bees in one hive and observed them. Their study was published in PLOS One.

"Pollen-stressed workers were less likely to waggle dance, and if they danced, the information they conveyed was less precise," Mattila said.

NASA prepares for final leg to Pluto

After a journey of 9.5 years across 3 billion miles of space, NASA's New Horizons spacecraft is about three months away from its closest approach to the dwarf planet Pluto.

The mission has gone according to plan so far, but NASA officials said hazards could emerge as the spacecraft plunges deeper into the Pluto system.

"This is no simple flyby," Jim Green, director of planetary science at NASA, said during a panel discussion in Washington. "We are flying into the unknown."

When the New Horizons mission got the green light in 2001, astronomers only knew about Pluto's largest moon, Charon, which is about the size of Texas. Since then, four more moons of Pluto have been discovered: Hydra, Nix, Styx and Kerberos.

There may even be more than that. Alan Stern, principal investigator for the New Horizons mission, said it is possible that there are other satellites around Pluto that are too small to be seen by telescopes in and around Earth.

New Horizon's closest approach to Pluto is scheduled to occur in mid-July.

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