Imagine building an entire castle out of LEGO bricks. No matter how grand the structure, it all comes down to those tiny individual blocks. Atoms work the same way for everything in the universe — every grain of sand, every drop of water, every breath of air is built from atoms. The word itself comes from the ancient Greek "atomos," meaning "uncuttable," because early philosophers believed atoms were the smallest possible pieces of matter.
At the center of every atom sits a dense nucleus, packed with positively charged protons and neutral neutrons. This nucleus is incredibly tiny — if an atom were the size of a football stadium, the nucleus would be a marble on the 50-yard line. Orbiting around this nucleus is a cloud of negatively charged electrons, whizzing around at mind-boggling speeds. It is these electrons that determine how an atom interacts with other atoms, forming bonds, conducting electricity, and making chemistry happen.
Here is a staggering perspective: a single drop of water contains roughly 5 sextillion atoms (that is 5 followed by 21 zeros). Yet each of those atoms is mostly empty space. If you removed all the empty space from every atom in every human on Earth, the remaining matter would fit inside a single sugar cube. Atoms are the ultimate proof that small things can build an entire universe.