"Sound money" isn't one thing — it's a set of properties. Judge each form of money yourself, then see how your verdict compares. You'll learn the framework by using it.
The assessments below reflect a widely-taught framework, but reasonable people weigh these properties differently — that's the point of judging for yourself. Notice that no historical form scores "high" on everything, and that even strong forms carry real weaknesses. Bitcoin is rated honestly: high on scarcity and divisibility, but only medium on fungibility (its ledger is public, so coins can be traced) and on acceptability (still far from universal). A property checklist is a useful lens, not the whole story.
The six properties (scarcity, durability, portability, divisibility, fungibility, acceptability) are the standard characteristics-of-money framework taught in economics and used in BSA's Curious Path, Module 1. Historical facts referenced (first standardized coins in Lydia ~600 BCE; the cowrie-shell trade; the end of dollar–gold convertibility in 1971) are covered with sources on the companion History of Money page. Bitcoin's 21-million supply cap and 1e-8 divisibility ("satoshis") are protocol facts (Bitcoin developer docs). Education, not financial advice.