What REALLY makes a cell phone waterproof? You can set a phone in a glass of water, and it might work for a day or two afterward. Maybe even a week. But the only way to know for sure if a phone is waterproof is to see how it is built. Does it have a hydrophobic coating? Are the ingress points solidly built? How does the IP rating system work? JerryRigEverything analyzes each of these points on popular 'Water Resistant' Smartphones. less