What Causes the Cringe-Inducing Sound of Knuckle Cracking?

It’s one of the most annoying habits, but it’s also pretty mysterious. What actually causes the sound of cracking knuckles?

Raheem DeVaughn – Mr. Midnight

black eyed peas, shakira – girl like me

calvin Harris, The Weeknd – over now

Audrey mika – excuses

ac/dc – realize

Q – Alone

POLAR BEAR ON ICEBERG

Ibis Fight

A little bit of soul

This image of the Soul Nebula (IC 1848) combines 21.1 hours of exposures through Hydrogen-alpha, Oxygen-III, and Sulfur-II filters. This object resides in the constellation Cassiopeia the Queen.

Jellyfish in the ocean of space

Supernova remnant IC 443 has the common name the Jellyfish Nebula, and it lies at the top right of this image. The other, larger, nebula is IC 444. Both objects are in the constellation Gemini the Twins. Terry Hancock from Grand Mesa, Colorado

Seven sisters

The Pleiades (M45) is the most famous star cluster in the sky. It lies 444 light-years away in the constellation Taurus the Bull. The clouds of gas seen in this image are not connected to the stars, but are a dust cloud through which the cluster is passing. Dave Doctor from Las Cruces, New Mexico

Old friend

The Orion Nebula (M42) may be the most photographed deep-sky object of all. It’s a huge emission nebula and star-forming complex. In addition to the glowing hydrogen, however, the object also has regions of both reflection and dark nebulae. James Esch from Lovettsville, Virginia