Good metaphor for tradeoffs associated with a bigger safety net

To communicate ideas, we need metaphors.  Greg Mankiw has a good one in this mornings NY Times, Can America Afford to Become a Major Welfare State?

Providing a social safety net is like using a leaky bucket to redistribute water among people with different amounts. While bringing water to the thirstiest may be noble, it is also costly as some water is lost in transit.

In the real world, this leakage occurs because higher taxes distort incentives and impede economic growth. And those taxes aren’t just the explicit ones that finance benefits such as public education or health care. They also include implicit taxes baked into the benefits themselves. If these benefits decline when your income rises, people are discouraged from working. This implicit tax distorts incentives just as explicit taxes do. That doesn’t mean there is no point in trying to help those in need, but it does require being mindful of the downsides of doing so. ...  

most European nations use that leaky bucket more than the United States does and experience greater leakage, resulting in lower incomes. By aiming for more compassionate economies, they have created less prosperous ones. Americans should be careful to avoid that fate.