I'm not very scientific, so bear with me.
I had always thought it was the atom, but then learned that the atom is composed of neutrons, and protons and stuff like that. And then you have quarks and subatomic particles as well.
But if those named things exist, they also in turn must be made out of something smaller. If it exists, it must have mass, and therefore must be composed of something, right? So could this go on for infinity???