HOUSTON — The country’s fourth-largest city on Sunday night ordered millions of residents to boil their tap water, citing a morning power outage at a city water purification plant. The city’s school system also announced late Sunday that it would be closed on Monday, at minimum,... ...It wasn’t clear what caused the water plant’s power outage, why it took the city 10 hours to issue a boil water order or how soon the order could be lifted...