In a children's museum, I ran across this fountain. You can adjust the flow rate with a valve, visible at the bottom. At low flow rates, the sheet of water does more or less what you'd expect: it curves downward, eventually falling more or less vertically. When you increase the flow rate, a surprising thing happens: the sheet of water curls back in toward the center. What causes this? Whatever it is, it seems that it must overcome the additional outward momentum of the water and then some. It is hard for me to believe that it is a result of surface tension for this reason--why should surface tension be so much stronger at the higher flow rate? Is it an aerodynamic effect? I did a brief internet search and didn't find anything. [![what do I put here?](https://i.stack.imgur.com/kudYB.jpg)](https://i.stack.imgur.com/kudYB.jpg)