What is it that, with blue blood and three hearts, changes its skin to blend in with the rocks on the seabed?

What has cities without houses, forests without trees, and water without fish? Solve the riddle → Which railway spans seven time zones and is so long that you can eat breakfast in Europe and dinner in Asia during the same journey? Solve the riddle → What is it that cannot possibly be claimed as true, at the very moment it occurs? Solve the riddle → A memory that isn't in your head, but guides your feet on a bike and fingers on a piano. Solve the riddle → What flows upstream in history but always runs north, through the heart of the desert? Solve the riddle →