* Subatomic Particles: Atoms are made up of even smaller particles:

* Protons: Positively charged particles found in the nucleus.

* Neutrons: Neutrally charged particles also found in the nucleus.

* Electrons: Negatively charged particles that orbit the nucleus in a cloud-like region called the electron shell.

* Quarks: Protons and neutrons are themselves made up of even tinier particles called quarks. There are six types of quarks (up, down, charm, strange, top, and bottom), but protons and neutrons are made up of just up and down quarks.

* Leptons: Electrons are part of a group of particles called leptons. Leptons are fundamental particles, meaning they aren't made up of anything smaller.

Beyond Particles: Things get even more abstract at this level:

* String Theory: This is a theoretical framework that suggests particles might not be point-like but rather tiny, vibrating strings. It's a very complex idea with lots of debate in the physics community.

Important Note: The "smallest" thing is a difficult question to answer definitively. Our understanding of the universe is constantly evolving, and new discoveries could change our view of what's truly fundamental.