When he was deputy governor of the Bank of England, Paul Tucker was fond of saying that if it looks like a duck, walks like a duck and quacks like a duck, then it probably is a duck. He was talking about money market funds and banks.

Asset managers protested that, though money market funds might look a bit like part of the banking system, since they were major holders of short-term bank debt, they were actually yellow plastic objects usually found in baths. Though their arguments were correct, they cut little ice with central banks (and with European lawmakers, though that's another story).