The paper shows that when you drop, say, a marble, into a liquid, what happens next can actually produce a supersonic jet of air! A review of this work can be found here, and you can also get access to the actual paper in the link.
In the kitchen version of the experiment, the marble creates a crown-shaped splash and crater as it falls into the liquid. The crater deepens to the point at which the walls start to contract. This is due to both the weight of the water outside and possibly surface tension, both of which create pressure gradients that force the collapse. Air inside this collapsing neck must escape upward or downward as the neck approaches pinch-off. It is in this escaping air that Gekle et al. found supersonic velocities—the first jet in this simple experiment.
A video of this also accompanies the review article.
I often wonder if the fun and fascinating tidbits of apparently "mundane" things like this is the reason why I got into physics in the first place. I know many people cite trying to understand the universe, or wanting to find the meaning of life, etc... etc. as the reason they study physics. I often find that I don't have such grand ambition. Instead, I find delightful pleasure in figuring out if quantum effects causes a pencil balanced on its tip to fall over, or if warm water freezes faster than cold water! Maybe I have a small mind....