Health Grades 7-9

Inside you, a daily battle is being waged and your immune system is at the frontline. Most of the time, you may not even notice it's there, but over the course of your life your immune system will guard you against hundreds of potentially fatal threats

Today, glasses help millions of people with poor vision be able to see clearly. But how? Andrew Bastawrous and Clare Gilbert help unravel the answer by explaining refraction — the ability of a transparent medium, like glass, water, or the eye, to change the direction of light passing through it.

Over our lifetimes, our bodies undergo a series of extraordinary metamorphoses: we grow, experience puberty, and many of us reproduce. Behind the scenes, the endocrine system works constantly to orchestrate these changes. Learn more here!

Katharina Ribbeck reveals the mysteries of this transparent, shape-shifting layer that moistens and lubricates the vulnerable parts of our body while rendering us invisible to potentially deadly microbes.

You're lifting weights. The first time feels easy, but each lift takes more and more effort until you can’t continue. Inside your arms, the muscles responsible for the lifting have become unable to contract. What’s going on?

Halitosis is a curse that has plagued humanity since ancient times. But what causes it, and why is it so universally terrifying?

Most of us don’t need more than one whiff to identify that generally unpleasant, characteristic smell we call body odor. But it’s a surprisingly complex phenomenon, influenced by our genetic makeup, age, diet, and hygiene. So what is this odor, exactly?

In about half of the human population, yeast in our hair causes dandruff. So, why do some people have more dandruff than others? And how can it be treated?

Fortunately, doctors today don’t resort to power tools to cure headaches, but we still have a lot to learn about this ancient ailment. Dan Kwartler shares what we know (and don't know) about headaches.

A throbbing, pounding headache. Bright zigzagging lines across your field of vision. Sensitivity to light, lingering fatigue, disrupted sleep. While an incapacitating headache is one of the most common symptoms, a migraine can include any of these experiences. So what exactly is a migraine? And what causes it?

If you’ve never had a nosebleed before, the first one can be terrifying. Why is blood pouring out of your face?

Water is essentially everywhere in our world, and the average human is composed of between 55 and 60% water. So what role does water play in our bodies, and how much do we actually need to drink to stay healthy?

Today, about one-tenth of the world’s population are left-handed? Daniel M. Abrams investigates how the uneven ratio of lefties and righties gives insight into a balance between competitive and cooperative pressures on human evolution.

The human voice is capable of incredible variety and range. As we age, our bodies undergo two major changes which explore that range. So how exactly does our voice box work, and what causes these shifts in speech?

You know that little pink thing nestled in the corner of your eye? It’s actually the remnant of a third eyelid. In humans, it’s vestigial, meaning it no longer serves its original purpose. There are several other vestigial structures in the human body, quietly riding along from one of our ancestor species to the next. But why have they stuck around for so long?

Most of us are no stranger to an occasional case of the hiccups. But what causes these ‘hics’ in the first place?

Flatulence is a daily phenomenon. In fact, most human beings pass gas 10-20 times a day (yes, that includes you). Where does your bodily gas come from?

There are a number of scenarios that can make us sweat–including exercise, eating spicy foods, and nervousness. But how does this substance suddenly materialize, and what exactly is its purpose? John Murnan explores the science behind sweat.

Why do we need a new flu shot every year? Vaccines for diseases like mumps and rubella offer a lifetime of protection with two shots early in life; what’s so special about the flu?

Some people love the feeling of cracking their knuckles, while others cringe at the sound. But what causes that trademark pop? And is it dangerous? Eleanor Nelsen gives the facts behind joint popping.

Teeth. We’ve all got ‘em (most of us, anyway). But how do they grow? Teeth are made from some biological nanotechnology that will blow your mind. They are strong enough to last hundreds of millions of years.