To understand the brain, you need to know how neurons are wired. Usually, they're activated one by one with light, like checking every faucet in a huge garden to trace the pipes. Millions of trials take time and can damage living tissue. The OPhELIA algorithm works like a seasoned gardener: it turns on the water at a few points and, from where it emerges, pieces together the irrigation layout. At its core is entropy, a measure of uncertainty: the algorithm selects neurons that deliver the most new information. It even learns from spontaneous activity, like spotting leaks. This way, with only 5% of stimulations in zebrafish, an almost complete connectivity map was recreated—20 times faster than usual. This approach is like how astronomers see dark matter by watching stars move, and it provides a standard model, a solid foundation for future discoveries.