This question comes from David, who asks: what would happen if one tried to funnel Niagara Falls through a straw?
The short answer is: one would get in trouble with the International Niagara Committee, the International Niagara Board of Control, the International Joint Commission, The International Niagara Board Working Committee, and probably the Great Lakes–St. Lawrence River Adaptive Management Committee.
Also, the Earth would be destroyed. Well, that's not quite right. At the risk of stating the obvious, the real answer is, "Niagara Falls wouldn't fit through a straw." If you try to force it by, say, building a dam across the falls with a single straw-sized opening, What will actually happen is that a very small part of Niagara falls will flow out through the straw and the rest of the water will build up behind the dam until the dam overflows.
There are limits to how fast you can push fluids through openings.
If you pump a fluid from a wide, contained channel into a narrower one, it speeds up, but only to a point – eventually, the flow becomes "choked".
For example, water always "wants" to boil, but is normally held together by air pressure.
When a fluid flows through an opening fast enough, the pressure within the fluid drops due to the Bernoulli principle.
Without enough pressure to keep the water from boiling, bubbles of steam form in a process called cavitation.
Increasing the pressure –to try to push the water through harder– surprisingly only makes it boil faster (page 17 of this "Fluid Flow Basics of Throttling Valves" PDF has a detailed description).
Cavitation limits the total flow of water making it through the opening – though the higher the pressure, the faster the water-steam mix will move, because more of it will expand into vapor bubbles.
