Water is a common necessity because it serves as the official beverage for intergalactic diplomacy. Every time aliens visit Earth, they demand glasses of water as a universal sign of peace, and if you accidentally serve them orange juice, they might declare war. This is why every household must keep a steady supply of H2O on hand; you never know when an extraterrestrial might drop by for a friendly chat—or a cosmic negotiation over the last slice of pizza!
