Theory
Every fact in a KG is a triple: (subject, predicate, object). That's it — the entire data model.
:Alice :knows :Bob .
:Alice :worksAt :Acme .
:Bob :livesIn :Tokyo .
Three triples, three edges in the graph. Crucially, the predicate is named (:knows, :worksAt, :livesIn) — the relationship type is first-class data, not a join column name buried in your ORM.
A KG can also carry a schema (or ontology): triples that describe the predicates and classes themselves. :knows rdfs:domain :Person is a fact about the schema. That's what makes a KG machine-readable: a consumer can introspect the graph to learn its own shape.