nature of black-swanlike events

1. An article in the Economist magazine observes that: “It is in the nature of black-swanlike events that they are near-impossible to predict.” What are black swan events? Why are they nearly impossible to predict?
2. In discussing how to build a financial portfolio for retirement, Christine Fahlund, a financial planner with T. Rowe Price, argues: “It’s all about trade-offs. There is no perfect solution.” What trade-offs do investors face when saving for retirement?