Science has been able to shed light on many of life’s mysteries over the centuries, offering explanations for diseases, animal behavior, the cosmos, and more.

We’ve come a long way from the days when life forms were thought to appear through spontaneous generation and bloodletting was used to cure almost any illness. But there still remain many scientific mysteries embedded in our daily lives. Here are five common occurrences that continue to defy explanations from the top scientific minds.

How Acetaminophen Works

You’d think that the accessibility of acetaminophen (Tylenol) as an over-the-counter painkiller would indicate a full understanding of its medicinal properties, but Big Pharma is still trying to figure this one out. Certainly scientists know the dangers of excessive doses, but exactly how the medication works to ease pain is still a mystery. It was once thought that acetaminophen functioned in the same manner as nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen, which block the formation of pain-producing compounds in the central nervous system. However, further testing indicated that this enzyme suppression only happens under certain chemical conditions in the body. Other researchers have examined the effects of acetaminophen on neurotransmission in the spinal cord, but a definitive mechanism remains elusive.

Why Cats Purr

This one’s easy – cats purr because they’re happy you’re petting them, right? Except they also purr when they’re hungry, nervous, or in pain, so there are more complex matters to consider. One theory put forth by bioacoustician Elizabeth von Muggenthaler suggests that purring functions as an “internal healing mechanism,” as its low-frequency vibrations correspond to those used to treat fractures, edema, and other wounds. Additionally, since humans generally respond favorably to these soothing sounds, it’s possible that purring has evolved, in part, as a way for domesticated kitties to interact with their owners. And researchers at least believe they now know how purring happens – a “neural oscillator” in the cat brain is thought to trigger the constriction and relaxing of muscles around the larynx – so it may not be long before they home in on more precise reasons for this common, but still mysterious, form of feline communication.

How Bicycles Remain Upright

It’s one of the great ironies of life that we supposedly never forget how to ride a bicycle yet lack a firm understanding of the mechanics that enable us to pull it off in the first place. Early attempts at rooting out answers gave rise to the “gyroscopic theory,” which credits the force created by spinning wheels with keeping bikes upright. This theory, however, was disproven in 1970 by chemist David Jones, who created a functional bike with a counter-rotating front wheel. Jones then floated his “caster theory,” which suggests that a bike’s steering axis, pointing ahead of where the front wheel meets the ground, produces a stabilizing “trail” similar to a shopping cart caster. However, this theory also has holes, as researchersdemonstrated in a 2011 Science article showing that a bike with a negative trail – a steering axis pointing behind the wheel – could maintain balance with proper weight distribution. All of which goes to show that while biking is largely a safe activity, there remains a glaring question mark at the heart of a $54 billion global industry.

How Animals Migrate

Maybe you’ve seen flocks of birds flying overhead to mark the changing of seasons or read about salmon fighting upstream to return to their birthplaces, but exactly how do these animals navigate in the midst of long distances and shifting geological conditions? In some cases, there are strong olfactory senses in play; a salmon can detect a drop of water from its natal source in 250 gallons of seawater, helping to guide the way “home.” But the possibilities get even stranger, as scientists are exploring the concept that light-sensitive proteins in the retinas of birds and other animals create chemical reactions that allow them to “read” the Earth’s magnetic field. It may seem far-fetched to think that birds rely on principles of quantum mechanics, but there may be no better explanation for how, say, the Arctic Tern stays on target while annually migrating more than 40,000 miles from pole to pole.

Why We Sleep

Given that we can pinpoint the health benefits and problems associated with proper and insufficient amounts of sleep, it’s baffling that we still don’t fully understand what this all-important restorative state does for the body. Older theories followed the notion that sleep helps people conserve energy while keeping them away from the dangers of the night, while more recent research explores how sleep contributes to the elimination of toxic neural buildups andpromotes plasticity, the brain’s ability to adjust and reorganize from its experiences. Other experts hope to come across answers by studying glia cells, which are abundant in the central nervous system and possibly involved with regulating when we nod off and awaken. And if these diligent researchers ever do crack the code of what sleep does for us, maybe it will shed light on related nighttime mysteries — like why we dream.