How do you tie water in a knot? First you make parts of it into vortices, which move more like long continuous strings than groups of autonomous molecules. Then you need to tangle those strings together.
Scientists have known that water can be tied in a knot for some time, and actually succeeded in "tying the knot" in 2013. Before, we saw computer images of what happened to that knot, but in this video we get a shot of the actual knot being tied in the water. The researchers, working the University of Chicago, explain that to make the knot they used, "a new method of accelerating specially shaped hydrofoils." Let's take a look!
The first part of the researcher's video shows a vortex cannon. That might sound impressive, but it's really just a hole through which a rush of water (or gas) can be forced all at once. When you blow smoke rings with your mouth -- that's a vortex cannon.
By watching the bubbles, we can see how it works. The water at the edge of the round hole curls around in circles, making a series of whirlpool -- or vortices. These vortices form a ring. The whirling motion makes the water forming the ring behave like string. It can be stretched and twisted, but each part of it retains a connection to the adjacent part.
This is relevant when we see the next part of the video. We see a solid structure, looped into a knot, get dragged into the water. You can't tell from the video, but a diagram of a cross-section of this structure shows that the "knotted rope" isn't round. It's a hydrofoil, shaped like an aeroplane's wing, and water flows around it like a wing. As the water comes off the wing end of the wing, it curls around, just like it did with the vortex cannon -- except this time instead of the vortices forming a ring, they form a knot. The overall shape of the knot distorts, but for quite some time, it remains in the shape of a knot, and behaves like an entangled piece of string. The first water knot!