Children’s voices pierce the air. A boy kicks a ball high before sprinting around a painted kickball diamond. Four square games host lines of students, chatting away as they await their turn. The swings creak, their seats full.

One girl sits alone with a book. She is immersed in the story, oblivious to her surroundings. Is she socially impaired? Mentally ill? Does she need medication?

Perhaps she’s just introverted.

Introverts are often categorized as shy and antisocial, but many have plenty of self-esteem and don’t fear interacting with others. What defines introverts (and extroverts) is the way they obtain and expend energy. Kate Bartolotta explains this well in her Huffington Post article “What Is It Really Like to Be an Introvert?” She compares a person’s energy stores to a cup. Each moment an introvert spends with other people, a little more energy is taken from the cup. Once their cup is empty, introverts need some alone time to refill it.

The opposite is true of the other end of the spectrum. Extroverts need to spend time socializing to fill their cups, and they become drained when they are alone. Most people identify with one type; however, no one can be completely an introvert or an extrovert. Carl Jung, the psychologist who popularized these terms, said such a person “would be in the lunatic asylum.”

The amount of time introverts need to spend alone depends on the amount of energy they can hold in their cup. Some introverts can socialize often, to the point where they may be mistaken for extroverts. But another’s cup may hold a smaller fraction of energy, so interaction tires them more quickly. These people might want to take solitary trips to restaurants or libraries, or stay at home rather than interacting with others.

In her book Quiet: The Power of Introverts in a World That Can’t Stop Talking, Susan Cain writes about Steve Wozniak, a cofounder of Apple. Wozniak worked tirelessly on what he called “The Dream.” His dream was to make a computer that used a screen and a keyboard and was small enough to use at home. He spent all his time working on this prototype. He labored alone and loved it, recalling it as “the biggest high ever.”

Bill Gates, another introvert, founded Microsoft and became a billionaire. Rosa Parks, a quiet, unassuming person, is credited with launching the civil rights movement with her act of bravery aboard a bus. Even successful actors like Audrey Hepburn have described themselves as introverts.

An important part of these famous introverts’ stories would be missing if we focused only on one end of the spectrum. The fact that Wozniak met extrovert Steve Jobs is crucial in explaining the founding of Apple. Had it not been for Jobs, Wozniak would have had a more difficult time bringing his dream to the world. Parks met Martin Luther King Jr., who helped raise awareness for civil rights by making groundbreaking speeches to huge crowds. It’s where the ideas started – in solitude – that matters.

Introverts aren’t necessarily smarter than anyone else, but to redirect them from the world inside their head to the party going on outside could rob the world of great advances. Everyone has been created differently, to act differently, think differently, and express differently than everyone else. It’s what makes us individuals, helping to shape our collective future – the future that little girl quietly reading her book will create.