Last week, the Chinese demonstrated that they have Donald Trump’s number. In perhaps the first case of open-ocean “trolling” for an internet response, Beijing managed to accomplish two things nearly instantly: provoke an immediate Twitter response from the president-elect and demonstrate how easily Trump can get spun up over a single tiny, low-tech, cheap piece of US military hardware.

And it all happened over an unarmed underwater drone. Six feet long and resembling an oversize child’s bathtub toy, the drone is meant to help the US Navy figure out how to use its actual submarines more efficiently, especially in shallower waters. It costs just $150,000, a pittance for the Pentagon. To put it mildly, it’s not the sort of US military equipment that usually triggers an international incident.

But it’s done so all the same. China seized the drone last week, claiming as an excuse that it was a “hazard to navigation” in the open ocean, and that they did so for safety reasons. The Pentagon insisted that it was operating in international waters — pointing out that the mothership was literally in sight at the time the Chinese grabbed the drone — and immediately demanded its return. Beijing offered to do so, only to have Trump, in a very Trump-like fashion, more or less accuse China of piracy and tell Beijing to simply keep it. The underwater drone was finally returned to the US on Monday night.

It is no surprise that the Chinese regularly “test” each new American president by manufacturing a potential national security crisis. George W. Bush had to deal with the Chinese “accidentally” bumping of one of our USN P-3 Orion planes, forcing it to land on Hainan Island, China, in April 2001, just a few months after he took office. Barack Obama saw swarms of small boats playing the maritime equivalent of “chicken” with some of our naval assets not long after he took office. The difference this time is that the timeline has accelerated, because this one was quite obviously aimed at Donald Trump, and he is not even in office yet.

So what’s really going on here? Do the Chinese have a point? And just what is an underwater drone anyway?

The drone China seized doesn’t carry weapons. That doesn’t mean it matters any less.

It’s no secret that the Obama administration loves drones. Armed unmanned aircraft like the Predator have become the president’s weapon of choice in the war on terror, targeting militants from Pakistan to Yemen to Somalia.

The drone China seized is very different. To begin with, it’s not a weapon at all.

The technology behind the Navy’s underwater drones is still so new that they don’t even have a proper military acronym yet. Some refer to them as AUVs, for autonomous underwater vehicle, others call them UUVs, for unmanned undersea vehicle, and there are at least half a dozen other names thrown around. Jargon aside, it’s easiest to think of the unmanned submarines as underwater gliders.

Before we go any further, it should be noted that there are other types of submarine drones. Several navies and oceanographic research institutions have a variety of shorter-endurance, shorter-range, and often faster submarine drones. In a real sense, most modern torpedoes could be called “UUV”s because they can operate on their own after being launched at their targets.

But those drones all require power, and a lot of it, to turn their propellers. That allows them to move much faster, but the trade-off is that they can’t go as far or operate for as long.

Underwater gliders like the one China seized are designed to get around that problem by using changes in water temperature to make the vehicle rise or sink, but to always move slowly forward. That means they can operate over enormous distances and for long amounts of time without needing to be recharged.

The drone itself relies on an idea first developed by an oceanographer named Henry Stommel and a maritime engineer named Doug Webb. Stommel died in 1992, but Webb founded a company that had by 2005 constructed a working prototype that could cover serious distances.

Named a “Slocum” (after the first man to sail around the world single-handedly, a badass named Joshua Slocum), a refined version of this design operated by Rutgers University was the first autonomous vessel to cross the entire Atlantic Ocean, though it took 221 days to do so.

By then, of course, the US military was already salivating, and in 2010 our Navy started fielding its first batch of drones using that same design from a company named Teledyne Marine (which bought Webb’s company in 2008). The US Navy version quickly added a package of sensors designed to help them map out not just the ocean floors but the ever-changing nature of the seas themselves.

Known as the “littoral battlespace sensing glider” (designated the “LBS-G” because of the military’s acronym-mad culture), the US already fields more than 50 of the drones. They are fairly cheap, with a reported price of about $150,000, so we are not really talking about big bucks from the Pentagon’s budget here.

To really understand Washington’s current standoff with Beijing, it is also useful to understand just why we have so many of these drones, why we want even more, and what purpose they might serve.

The Pentagon uses these underwater drones to prepare for a war it hopes won’t come

Officially, these underwater gliders collect entirely unclassified data on water temperature and salinity along their route. Why does that matter? That’s the real question, and it all comes down to the hidden world of submarine warfare, which is all about the physics.

The Navy wants more of these cheap little subs because the very simple, basic water information they provide can help American submarines hide, or hunt, in deep water or shallow. And in shallow waters, like the South China Sea, where the drone was seized, knowing simple things like how salty or warm the water is really matters. The Navy uses that information to help sort out where and how enemy submarines might be operating.

Those models would provide a huge advantage in the case of potential combat with China should the new administration come to blows with Beijing in the South China Sea.

All of that explains why China might want one of these drones, and why it's so important that the US got this one back. On the other hand, it’s equally likely that China is just screwing with the president-elect, since all of this technology is freely available on the open market. These types of UUVs are used by universities, environmental groups, and a whole host of other civilian consumers. So it’s not like they or their data is secret.

As the New York Times observed, American allies across Asia are confused and alarmed by the Obama administration’s failure to respond strongly to the seizure by, for example, dispatching an American destroyer to the area near where the drone was snatched. The newspaper noted that the drone was captured not only in international waters but also beyond the marker China uses to delineate its claims in the South China Sea.

Beijing, in other words, was effectively acting as if all of the South China Sea belonged to it, and the US didn’t push back with force. It’s hard to imagine a cheap and low-tech drone dealing a serious new blow to the already fragile US-China relationship. But it did all the same.