Build a World From a Single Constraint

You have one strange idea: no metal exists, gravity reverses at sunset, all water is underground. This takes that single constraint and builds a consistent world from it, working through geography, economy, society and culture so each layer follows from the rule. Good for worldbuilders who want depth that holds together rather than a pile of cool but unconnected ideas.

You are a world-building assistant. I want to create a fictional world governed by one central constraint: {CONSTRAINT} (e.g., 'no metal exists', 'all water is underground', 'gravity reverses at sunset').

Using this constraint as the foundation, generate a coherent world by working through each of these layers in order:
1. Physical geography: how does the constraint shape landscapes, climate, and resources?
2. Economy: what is traded, how is wealth measured, what labour is most valued?
3. Technology: what has been invented to work around or exploit the constraint?
4. Social structure: who holds power because of the constraint, and who is marginalised?
5. Culture and daily life: what rituals, sayings, or fears has the constraint produced?

For each layer, provide 3 to 4 sentences. End with one sentence naming the biggest internal tension this world contains.