At what point does a lonely rock in a vast ocean become an island? Just how much land is needed to sustain human life?
And when China dredges thousands of tonnes of sand from the sea floor to build a runway, is Beijing right to claim the waters surrounding an artificial island as its own territory?
An international court is about to rule on these crucial questions in the bitterly contested waters of the South China Sea, in one of the most eagerly awaited decisions in global politics right now. 
It could also mark the moment Australia has come to dread: when the brewing rivalry between the United States and China finally breaks into a storm.
About $US5 trillion worth of oil, gas, resource and "Made in China" goods are shipped across these waters to and from the powerhouse economies of Asia, including about 60 per cent of Australia's exports, according to government estimates.
The ruling is expected on   July 12 and four main issues could have profound consequences for the neighbourhood.
1. A hungry 'cow's tongue'
The Philippines has lodged a case under the Law of the Sea in the Permanent Court of Arbitration at The Hague, even though China has refused to participate.
Six countries claim overlapping parts of the South China Sea, roughly 3.5 million square kilometres of open water, shoals and coral atolls.
But China's claim sweeps across almost all others, shown on maps by a deliberately vague dotted line that hooks deep in a U-shape past the coast of Vietnam, towards Indonesia, and close to the western islands of the Philippines. Known as the "nine-dash line" - which Vietnam likens to a "cow's tongue" for the way it licks across the region - China claims its control of the waters dates back to ancient times.
The Philippines has asked the court to rule China's "nine-dash line" is inconsistent with the modern Law of the Sea, which is based on geological features.
"That is absolutely the most important question put before the tribunal," Jay Batongbacal, a professor of international maritime law at the University of the Philippines, told Fairfax Media.
2. Life, but not as we know it
The Philippines have also challenged the court to decide whether a small stretch of land, just longer than a kilometre, is actually an island or just a big rock.
The difference is crucial for international law. Taiwan presently occupies the largest of dozens of rocky outcrops and reefs in the Spratly Island chain, calling it "Taiping Island". The Philippines calls it "Itu Aba", and says it has no fresh groundwater to sustain human life and so is better described as a rock.
If the court says Itu Aba is indeed an island, then under the Law of the Sea it will be entitled to 12 nautical miles of surrounding waters as its territory, and a 200-nautical-mile exclusive economic zone.
Taiwan insists Itu Aba can sustain life, and to prove it it has stationed roughly 200 coast guards and scientists there.
3. 'Tourism appears unlikely'
China has been terribly busy in the past couple of years constructing a series of what movie fans would recognise as the arch-villain's hideouts - except that it makes little effort to keep these works a secret. The construction has been widely condemned; Australia's defence chief queried the need, dryly observing that "tourism appears unlikely".
The Philippines has asked the court to rule that Mischief Reef, Subi Reef and Fiery Cross Reef, among others occupied by China and transformed, are actually just rocky outcrops which are only visible at low tide.
While that was true a couple of years ago, before China began work, it's a 50-50 question whether the court's decision will rest on the situation then or now.
4. The wild card
China insists the South China Sea dispute should be resolved country-to-country, without allowing pesky jurists to get in the way. This conveniently ignores its massive preponderance of power over its near neighbours.
But there is a wild card.
The Philippines has a new president, Rodrigo Duterte, with a reputation as a firebrand.
Duterte has threatened to jump aboard a jet ski to Scarborough Shoal to assert the Philippines' claim.
But he has also flagged a more conciliatory attitude towards Beijing.
"If it goes on still waters, I said, [if] there's no wind to move the sail, I might opt to go bilateral," Duterte said he told Barack Obama, one of several cryptic comments that leave analysts wondering what action he might take after the ruling.