What's the Smallest Thing in the Universe?

The punch line probably involves a Planck length

The answer to the enduring question of the smallest thing in the universe has evolved along with humanity. People once thought grains of sand were the building blocks of what we see around us. Then the atom was discovered, and it was thought indivisible, until it was split to reveal protons, neutrons and electrons inside. These too, seemed like fundamental particles, before scientists discovered that protons and neutrons are made of three quarks each.