Zero

Zero is the number that means "none" or "nothing." It is written as 0. Zero sits between the negative numbers and the positive numbers on a number line. It is the only number that is neither positive nor negative.

Zero might seem like an obvious idea, but it is not. For most of human history, people did not have a number for nothing. Why would you count something that is not there? Ancient Greek and Roman math worked fine without zero for thousands of years. The Romans wrote big numbers like MCMLXIV without ever needing a symbol for empty space.

The problem with no zero is that big numbers get messy. Look at the number 305. The 3 means three hundreds. The 5 means five ones. The 0 in the middle means there are no tens. Without a zero, how would you tell 35 apart from 305? You would have to leave a blank space and hope nobody got confused.

The first people to use a symbol for an empty place were the Babylonians in Mesopotamia, around 300 BCE. The Maya in Central America also invented their own zero, hundreds of years later. But neither group used zero as a real number you could add or subtract. It was just a placeholder.

The big breakthrough came in India. By the 600s CE, an Indian mathematician named Brahmagupta wrote down rules for using zero in math. He explained what happens when you add zero to a number, subtract zero, or multiply by zero. From India, the idea spread to the Arab world, and from there to Europe. Europeans were slow to accept it. Some cities even banned the new number as suspicious.

Zero has a few strange habits. Adding zero to any number leaves the number the same. Multiplying any number by zero gives you zero, every time. But dividing by zero is not allowed at all. Mathematicians say the answer is "undefined," because no answer makes sense.

Today, zero is everywhere. Computers run on a code of just two digits, 1 and 0, called binary. Every text message, photo, and video on your phone is stored as long strings of ones and zeros. A number that once seemed like nothing turned out to power almost everything.