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Hank: Kids... are weird, and not just because they do a lot of weird things like stick crayons up their noses or eating dirt. Kids are weird because humans go through a lot of changes between infancy and adulthood. Like that whole baby teeth thing. There’s a lot going on in your body in the first couple of years that you're alive and some of those things are pretty freakin bizarre.
 
Many babies are born with blue eyes and they can change color until they settle on a more permanent hue. Eye color is influenced by pigmentation of the outermost regions of the iris called the stroma. In this case it's a pigment called melanin, which also affects your hair and skin color. The specialized cells that produce and store melanin are called melanocytes. And melanin reflects and absorbs light in different ways depending on the amount of pigment and the distribution of melanocytes in the iris, which leads to different eye colors. Blue eyes have almost no pigment, green or hazel eyes have some yellow and brown pigment, and brown eyes -- you guessed -- it have lots of brown pigment.
 
Many newborn babies, especially those of Caucasian descent who generally have lower levels of melanin throughout their life, don't have that much pigment in their eyes, making them look blue. Over time, in some people the melanocytes will begin to produce more pigment leading to a slow shift in eye color throughout childhood. This usually settles down by around age 5, but some people may change eye color later in life as well. There’s a lot of variation because scientists estimate that around 16 genes may play a role in determining eye color.
 
In utero, your brain actually over-produces brain cells called neurons through a process called neurogenesis. Before you're born nearly half of those brand new neurons will die off naturally through a process called apoptosis, basically programmed cell death. Scientists think that apoptosis is a really important part of helping your brain make the right neuronal connections to function. All cells have the program, and your brain uses protein signals to keep it turned off or turn it on.

For example, when a developing neuron grows toward the right cell and makes a connection called a synapse, it receives some proteins called neurotrophins, which protect against apoptosis. Neurons that don't make the right connections don't get these neurotrophins and die off instead.
 
By the time you're born your brain is mostly done with neurogenesis and apoptosis, but the wiring isn't finished yet; your brain continues to fine-tune these synaptic connections throughout your childhood. As a kid, you have way more neuronal connections than you need. Connections that are used the most will be stabilized and protected so your brain can keep signaling with them, and the extra connections that go unused were pruned back by the brain's immune cells called microglia.
 
The microglia wrap around these extra connections and break them off of the neuron, gobbling up the cell debris. This helps the neurons focus on maintaining their stable, highly used synapses, and makes way for new synapses to connect and develop.
 
When you're born, your head is enormous compared to the rest of your body. An infant's brain is already a quarter of the size of an adult brain, but their body mass is much much smaller than the mass of an adult. And by the time you’re two, your brain is already about 75% the size of a fully-grown adult brain. Now this might seem strange since we talked about how your brain doesn't grow many new neurons after you're born. Turns out your brain keeps producing other kinds of brain cells collectively called glia, to help support and maintain the neurons.

But how can your brain grow so much when your skull is surrounding and protecting all that tissue? Well we've evolved these soft spots, or fontanelles, on our infant skulls, places where the skull hasn't yet fused together, leaving it flexible so a baby's head can be compressed a little bit to pass through the birth canal. Compared to other primates, our soft spots fuse relatively late in childhood, at around two years old instead of just a couple months, allowing space for the brain to gradually grow and develop. So your giant baby head can expand to hold all of that growing brain mass, and the rest of your body can catch up to it later.
 
Human babies grow really slowly compared to other mammals. We have a prolonged childhood, and little kids require a lot of attention and supervision. It's kinda hard to imagine why staying little for so long could have an evolutionary advantage. Some scientists think they figured it out and it's because our brains use a lot of energy. Using brain imaging data, a team of researchers from multiple universities estimated how much energy our brains need at different points in our lives. They were specifically looking at how much glucose the brain was consuming and 29 kids, and compared it to the body's overall daily energy requirement, and the glucose consumption of seven young adult brains as a control.
 
Surprisingly, they found that even though our brains are proportionately largest right after we’re born, that's not when they seem to be using the most energy. When you're around five years old, right when your childhood growth rate is slowest, your brain uses energy at a rate of about two-thirds of your body’s resting metabolism. This is around the time when your brain is really stabilizing and pruning all of those neuronal connections. So scientists think your brain could be using so much energy doing this construction work that your body has to slow down other processes, like growing up. And once your brain's energy consumption starts to decrease again, you'll eventually hit the last big growth spurt, that fun friend we call puberty.
 
Everyone’s familiar with the sensation of shivering. Basically when you get cold your brain to start signaling your muscles to make them quickly contract and relax. The process produces heat and warms up your body. But babies under the age of around three months haven't developed the systems and muscle tissue needed to shiver yet, so they have to keep warm some other way.

They have more of a special kind of fat, called brown fat, with lots of blood vessels and energy producing cell organelles called mitochondria. So when babies get cold, they burn brown fat located in their chest, shoulders, and backs, this releases heat energy to warm them up and the whole process is called non-shivering thermogenesis. It’s essentially the same thing that helps keep some animals warm during extended hibernation. This is really important because babies are particularly susceptible to cold. They can't dress themselves, or move away from cold things, so they needed a good way to keep warm.
 
The larynx is the technical term for the voice box. It helps you breathe without inhaling your food by closing the epiglottis, the flap of cartilage behind the tongue, so food can slide right on past to the stomach. Plus the combination of vocal cords inside the larynx and air coming from the lungs allows us to produce speech. The human larynx is lowered down in the throat then in non-human primates and scientists think this anatomy helps us articulate so many different sounds. It’s pretty important for producing language, which I will talk more about in a second.

In infants, the larynx is positioned really high up, pretty much inside of the skull -- which is similar to non-human primates like chimpanzees -- and this positioning is really useful. It allows babies to breathe through their noses while eating. Since babies aren't very coordinated, this is really important for protecting their lungs from any liquids or food bits so they don't choke. But by two years of age the larynx has begun to drop into the neck, so the pathways to the lungs and stomach begin to intersect. As an adult, it's up to you to figure out how to safely breathe and eat at the same time.
 
Now this doesn't sound that interesting, after all we use language every day and of course you learn it as a kid, but that's what makes it so cool. We learn language without any formal teaching. Linguists think the basics of language: speaking, signing, and linguistic comprehension, are biologically inmate; basically all humans can use these skills. There are lots of different theories about how we learn language, and none of them are a really complete explanation. But scientists do think it has something to do with experiencing language in your environment.
 
Jean Piaget, a legendary child psychologist, was a fan of the cognitive theory. He believed that children first developed the ability to understand the idea of a thing, like the concept of a ball or the feeling of love, and then they learn how to express it in words. Another popular theory is learning through invitation and positive reinforcement. Children learn by imitating adults and getting feedback on what's right and what's not. But kids of course can make up completely new sentences and words that they'd ever heard before so this theory can't explain it all.

So there's another possibility: the innateness of certain linguistic features. The theories that all humans are born with some innate grammatical knowledge and just need exposure to a primary language to start putting it into action. This could maybe explain why all languages have things like vowels and consonants and nouns and verbs. If we're somehow hard-wired to understand and use them, even the very first languages could contain these basic kinds of tools.

So parents always say that kids grow up fast but it turns out there are a lot more weirder cooler things to childhood development than you might have expected.
 
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