The orange juice market is huge. Unfortunately some processors try to cut corners by extending the juice with sugar, pulpwash and water. But the ruse can be detected by some clever science. Oxygen in nature occurs as two possible isotopes, O-16 and O-18, which differ slightly in mass. The natural abundance of O-18 is only about 0.2%, but it is more abundant in water in growing plants because the heavier isotope is less likely to evaporate. So a juice diluted with water from non-biological sources will have a different isotope distribution and an instrument called a mass spectrometer can detect this. The authorities have already used this method to put the squeeze on some OJ fraud artists.