“Stringing” a 3-dimensional integer lattice of “beads” with a Hamiltonian path (one that crosses every vertex but only once) as a visual showcase of 1-1 correspondence with the integers.

This is a bit like the Aleph-Null version of how Space-filling curves were discovered after Cantor’s surprising proof that the continuum of unit interval has the same cardinality as that of the unit cube.