A lighthouse is a textbook public good: once its beam is lit, no ship can be excluded from seeing it, and one ship benefiting doesn't leave less light for the next. This simulator generates a population of citizens, each with their own private willingness-to-pay for the lighthouse, and lets you fund it two ways โ voluntary contribution, where rational free-riders chronically under-fund the good relative to what the population collectively wants, or compulsory taxation, which reaches the full collectively-desired funding level regardless of individual incentives. Watch the beam's brightness and range, and how many ships drift beyond its safe glow, respond live to the funding gap.