什么是 The structure ladder？

Matter is built in floors, each assembled from the one below: three quarks (pointlike, smaller than $10^{-18}\,\mathrm{m}$) bind into a nucleon — proton or neutron, about $10^{-15}\,\mathrm{m}$ across; nucleons pack into a nucleus ($10^{-15}\,\mathrm{m}$ to $10^{-14}\,\mathrm{m}$); a nucleus plus its electron cloud is an atom ($10^{-10}\,\mathrm{m}$); atoms bind into molecules and crystals ($10^{-9}\,\mathrm{m}$ up), which assemble into everyday objects ($1\,\mathrm{m}$), planets ($10^{7}\,\mathrm{m}$), planetary systems ($10^{11}\,\mathrm{m}$), galaxies ($10^{21}\,\mathrm{m}$) and the observable universe ($10^{26}\,\mathrm{m}$).