A long time ago, a smart man named Leucippus from ancient Greece had a big idea. He thought that everything, like water in a glass or the desk you sit at, is made of tiny bits called atoms. These atoms are too small to see and are always moving around.
Think of a box of really small LEGO pieces. Just like you can make many things using those LEGO pieces, atoms come together to make everything we see and touch. At times, they come together to make water, sometimes wood, or even metal.
Not many of Leucippus's books are around today, but he did write some, like "The Great World System." And here's the cool part: scientists today say Leucippus was right! Everything is made of atoms, just like he said.
