Complexity is the quality of having many interconnected parts that work together in ways that aren't immediately obvious. A simple toy car has four wheels and rolls forward, but a real car has thousands of parts (engine, transmission, brakes, electronics) that must work together. That's complexity.

You encounter complexity every day. A video game might look simple on the screen, but its code contains millions of instructions handling graphics, sound, player choices, and artificial intelligence all at once. Your body shows remarkable complexity: your brain, heart, lungs, and digestive system operate simultaneously, coordinating through nerves and blood vessels you never think about.

Complexity differs from just being complicated or confusing. A jigsaw puzzle with 1,000 pieces is complicated (it takes time and patience), but the solar system is complex (planets, moons, asteroids, and comets all moving in patterns shaped by gravity, with each object affecting all the others). Scientists study the complexity of ecosystems, where plants, animals, weather, and soil interact in ways that can surprise us.

When something has high complexity, small changes can create unexpected results. That's why building a treehouse is straightforward, but designing a skyscraper that can withstand earthquakes requires engineers who understand structural complexity.