More on the almost-emptiness that is black holes. Because they're invisible, they're difficult to find—but sometimes get discovered because they give off X-rays.
Even though it sounds totally crazy, astronomers are very confident that black holes exist. Our galazy is really old, it should carry at least 100 million black holes but we’ve found only a couple of dozen of them.
It used to be thought that a pathogen (or germ) and its host develop in a 'mutually benign relationship'—this was called the 'Law of Declining Virulence'. If the common cold killed us there would be less hosts and the virus would decrease. So the common cold virus mutated to become less lethal, and more common. The Law of Declining Virulence was debunked in the 1980s, and the pathogen/host relationship is actually pretty complicated.
Power steering on a car involves various rods of steel moving relative to one another. How the rods connect is why mechanics are dealing with an unexpected problem.