Mark Thompson says he likes to take on "big, juicy strategic puzzles" — first when he ran the BBC, then The New York Times, and now as CEO of CNN. The new job might be his hardest one yet: CNN is a big brand that still makes a lot of money, but it's a cable TV network, and cable TV networks are in big trouble . So Thompson is trying to find an answer in streaming, with a new service that will give you just about everything CNN shows you on TV, for $7 a month. The most obvious question about the