The collapse of gas bubbles in the joint synovial fluid causes 'cracking' knuckles.

See the anatomy of a healthy knee joint, which has smooth cartilage that cushions the joints and protects the bones from rubbing together.

A human joint is any place in the human body where two bones connect, providing structure and stability.

Kangaroos evolved exceptionally thick knee cartilage to absorb shock, and scientists are studying the animals for potential insights about how to safeguard human knees against arthritis.

Variations in rapidly expanding gas and mechanical elements produce fireworks sounds

Chemical compounds designed to vibrate rapidly when ignited produce whistling, screeching sounds, while crackles and pops are created by small star pellets inside the firework, which explode one after another quickly. Fluttering and chirping sounds are produced by spinning elements that rotate rapidly as the firework ascends.

To use the various chemicals in crude oil, they must often be cracked—broken down into smaller molecules—to make their processing easier in everyday machines.

Watch real knee surgery footage to see what arthritis looks like up close.

Explore the basics of arthritis in this 1440 Original video with orthopedic surgeon Dr. Nick Golinvaux.

Visualizing a joint under attack from rheumatoid arthritis

A particularly difficult arthritis subtype is rheumatoid arthritis, a painful condition provoking a vicious cycle of joint inflammation, destruction, and dysfunction. It begins with a misfiring of the patient’s own immune system—for unknown reasons, it identifies joint and cartilage cells as foreign, initiating an attack to eliminate the supposed "invaders."

What makes food crispy, and how science can be used to maximize this property

The fracturing texture experienced when biting into vegetable tempura, fried chicken, pastry crusts, and other foods comes from having a dehydrated, porous surface. When frying food in hot oil, trapped water vaporizes and escapes, creating tunnels and pores in the food as it dehydrates. Breading, particularly with the large, irregular shapes of panko, and leaveners can help create more air spaces.

Breaking down the knee's anatomy

The human knee is a remarkably complex structure carrying the full weight of the body throughout a person's life while allowing a number of complex twists, flexes, and extensions. It's also one of the body's biggest weaknesses—the location of chronic pain and replacement surgeries for millions of Americans.

Arthritis is an umbrella term for various conditions that cause joint pain, including osteoarthritis, inflammatory arthritis, posttraumatic arthritis, and septic (infectious) arthritis.

The most common type of arthritis is osteoarthritis, which is sometimes called 'wear and tear' arthritis, referring to age-related cartilage loss.

The bullwhip was the first human-made object to break the sound barrier, producing a small sonic boom heard as a crack when flicked quickly.

When struck, objects vibrate at unique rates called resonant frequencies, which makes tapping wood sound different from tapping metal, but providing objects with sound at their resonant frequency can cause them to shatter.

Cooling lava naturally creates hexagonal cracks to evenly relieve stress

When lava cools and turns into rock, it contracts and builds up tension, particularly when held in place by the surrounding landscape. Just as drying mud cracks, initial cracks in the rock are random, but quickly organize into a hexagonal pattern, which scientists consider the most space-efficient shape.

Arthritis affects almost 60 million Americans, with about 1 in 5 US adults developing this type of joint pain as they age.

Limb regeneration research may lead to treatments for arthritis

For scientists pursuing the cure for arthritis, the holy grail of discoveries lies in how to regrow healthy cartilage inside human joints. While this feat has so far resisted our best scientific attempts, there are plenty of other human tissues—such as the liver and fingertips—that will successfully regenerate.

What is a sneeze? (And can it kill you?)

A sneeze is an important involuntary reflex to expel something that is irritating your nose. But can it kill you? No, say these silly video hosts. Although, the sneeze can expel air, saliva, and mucus at nearly 100 miles per hour in a five-foot radius. See them break it down with this quick, quirky video.

Tennis balls get their bounce from pressurized nitrogen inside vulcanized rubber, made from heating natural rubber with sulfur to create molecular links that harden the material.

Diagnosis and treatment of hip and knee osteoarthritis

Osteoarthritis is the most common type of arthritis, resulting from age-related degeneration of the soft cartilage lining between bones. As this layer breaks down, the normal smooth motion between bones is lost and the joint becomes painful.

The surprising science behind Pop Rocks

Have you ever wondered why Pop Rocks makes tiny explosions in your mouth? Well, there's actually pressurized carbon dioxide bubbles trapped within the sugary candy. This 19-second video shows you what Pop Rocks look like under a microscope.

A closer look at why people snore

Snoring is simply noise caused by improper airflow during sleep—and while it’s something that most people will do at some point in their lives, it can also be indicative of larger health problems. Generally speaking, snoring occurs when soft...

Skull bone marrow may be playing a key role in chronic pain.

Scientific applications of bubbles include logic gates, laser production, and medical imaging

Microbubbles of nonreactive gas can be administered to improve the reflection of sound waves and the resulting imaging during an ultrasound. Bouncing light waves within a bubble can stimulate emission of light from a trapped chemical, just as occurs within a laser, and bubbles can act as valves that control the flow and reaction rates in microfluidic systems.

Impressionist painter Pierre-Auguste Renoir suffered from arthritis in the 1800s when there were no treatments, and eventually had such limited hand function that he painted with brushes placed in his mouth.

Crackling, popping, and rustling sounds have been reported during auroras

At the altitudes where auroras take place—between 100 and 300 kilometers (62 to 186 miles)—air is not dense enough to efficiently vibrate and carry sound waves to the ground. However, one theory suggests that the geomagnetic energy behind auroras can trigger electrical discharges near the surface, which may be the cause of reported sounds.

Through sonoluminescence, concentrated sound waves can produce short, high-intensity flashes of light by rapidly collapsing microscopic bubbles to temperatures up to twice that of the sun's surface.

There is no cure for arthritis, which includes more than 100 separate joint pain and stiffness conditions.

Collagen supplements have become a multibillion-dollar industry

Beginning in our 20s, the amount of this protein in our skin, muscles, and connective tissue declines. But independent, high-quality studies—like those that are double-blind—have not yet shown a significant association between their use and evidence of it improving skin elasticity, reducing wrinkles, or combating hair loss. Some collagen supplements do seem to help with joint pain and knee function, but NPR cautions that studies on that purpose are industry-funded.

Hear examples of what tinnitus can sound like to those suffering from it.

Why do we sometimes jolt awake before falling asleep?

Around 70% of people are estimated to experience the sensation of falling and jerking awake right as they're about to drift to sleep. That mysterious phenomenon is called hypnic jerking. As common as it is, hypnotic jerking remains disconcerting and hard to explain. While we don't yet have clear answers, peruse this article in Popular Science for some possible explanations as to why this happens.

Likened to a car backfiring or a faraway cannon fire, skyquakes have been reported worldwide in Earth's atmosphere without any obvious cause.

A standard glass of champagne poured straight down releases about a million tiny carbon dioxide bubbles, according to two decades of research by a French physicist.

In 1880, Jacques and Pierre Curie discovered that electricity could be produced by striking certain materials—the piezoelectric effect—a phenomenon that is seen in barbecue lighters.

Geode formation begins with trapped gas bubbles in igneous or sedimentary rock

Over thousands or millions of years, water seeps into the cavity created by the bubble and deposits minerals, which slowly build and crystallize into quartz, agate, amethyst or other crystals.

Tommy John surgery, which can save a pitcher's career, reconstructs a torn ligament in the inner side of the elbow that would otherwise stabilize the elbow during throws.

Learn how your body is, scientifically, like a firework with its own slow, controlled burn.

Five advancements in arthritis research

It is estimated that nearly 350 million people worldwide are currently suffering from arthritis. This resource explores five of the most promising recent advancements in arthritis research as scientists work towards improved treatments and continue the search for a cure.

Pickleballs' loud 'pop' sound can reach 120 decibels, equivalent to the sound of a hammer striking a nail.

When experiencing turbulence, fluid elements undergo stretching and folding

Simulations show patches of fluid particles, such as splashes of milk, bend under turbulent flow, creating curvature that grows linearly then exponentially. This accelerated deformation enhances folding and mixing faster than other methods, such as stirring or beating.

'Old people' smell is real—it's a chemical buildup when fatty acids on the skin oxidize

The chemical composition of body odor seems to change with age. Later in life, we harbor higher concentrations of a chemical compound called 2-nonenal that forms when omega-7 fatty acids on the skin oxidize, and the skin's natural antioxidant defenses weaken. Commonly called "nonenal", it's the smell widely associated with aging, and it's often described as a grassy or greasy odor.

The Gasbuggy Project detonated a nuclear bomb to fracture underground rock formations

Part of Project Plowshare—a program aimed at finding peaceful uses for nuclear devices—the 1967 experiment sought to extract natural gas trapped within shale in underground New Mexico. Gas radioactivity was found to be excessively high for use without expensive processing, and the project was deemed an economic failure.

Cyanoacrylate, or superglue, was discovered by accident during World War II when researchers tried to create a clear plastic for precision gunsights.

Listen to what a forest in the Jurassic Period may have sounded like with this reconstructed soundscape, which was created by analyzing the fossilized wings of 165 million-year-old insects.

Tectonic earthquakes are caused by tectonic plates sliding past one another at fault zones, deforming rock through friction as energy builds like a stretched rubber band.

Farts are tiny status reports from our gut microbiome, with American adults typically passing gas more than 30 times daily.

Thunder occurs when fast-flowing electricity heats and pressurizes air, causing it to expand and produce a shockwave at each point along the bolt at multiple frequencies.

Some scientists hypothesize that stressed rocks can discharge built-up static electricity in the form of lightning—a phenomenon known as fracto-luminescence, or earthquake lights.

Tornadoes produce inaudible signals to humans hours before forming

Researchers at the National Oceanic and Atmospheric Administration have found that tornado-producing storms generate specific sounds below the range of human hearing, much like the infrasonic signals produced by earthquakes and avalanches. Although difficult to distinguish from wind noise, scientists hope to build systems that use these signals to improve tornado forecasting.

The disease no one survives (yet)

Creutzfeldt-Jakob disease is currently incurable and can cause one's death within months of symptoms appearing. It involves misfolded proteins that, for some reason yet unknown to science, aren't cleared by the brain as typical and instead trigger similar proteins to misfold. These malformed major prion proteins—prion being a portmanteau of protein and infection—clump inside nerve cells, blocking signals crucial to processes ranging from speaking to organ function.