Re: Atoms, protons, neutrons and electrons.

Dear Ulysses,

That's one of the questions that particle physists like myself are trying
to answer! We don't know! Our current experiments tell us that quarks and
leptons are the smallest particles, but we have theories (mainly one called
string theory) that tells us everything is made up of even smaller
particles called strings. We don't yet have any experimental evidence for
string, but we're looking!

I hope this helps!