The answer?

"I can personify myself without much problem," said Melancholy. "All you have to do is to is imagine me. Everything exists in your imagination. Even God, alas, exists only in your imagination. But imagination is a material thing, and can even be measured. In 1936, an Englishman named James Sprague invented a very sensitive apparatus that made such measurements possible. At the same time the similar contraption was invented by another man, a Russian called Nikita Sniffkin, but he was from a poor village and fame passed him by."
  "All right," says Rosie, "So you're saying that God, who I've seen, only exists in my imagination. Does that mean when I'm no longer here He will cease to exist as well?"
  "By no means," said Melancholy, "because you in your turn exist in His imagination."
  "Then why is God God?" asked Rosie, "Why isn't it me that's God? The net result would be just the same!"
 "God is God because He has the greatest imagination of all of us. He believes that everything exists: the planets, the universe, and all the living creatures. This involves believing in a large quantity of shapes of great density, which keeps Him very busy and means He is exhausted, and has no time, for example, to learn Czech on top of his native Chinese and a couple of other languages."
   "But if everything exists only in imagination, why do the laws of physics function consistently? And what about maths? Are you trying to tell me that one and one doesn't make two?"
"One and one makes two, three, five or even seven in some cases... But at that point it gets too much like hard work, and people fall back on the simplest solution", said Melancholy and took her leave, „Let's not go into it. I've got some other work to do now."