Water spinach - also called ong choi, morning glory, kangkong and water convolvulus - is a vegetable that's occasionally in the news, although not necessarily for positive reasons. In the United States, it's considered an invasive weed because the plant grows quickly and clogs up dams and waterways.
Because the vegetable grows in water, it's subject to infection by bacteria and other microbes - so it's not a good idea to harvest it from a pond you happen upon during a hike.
Water spinach is distinctive for its pale green, long, hollow stems and darker slender leaves. When buying water spinach, look for lots of tender leaves on relatively short stems. You'll see piles of water spinach in markets now; the vegetable is believed to have a cooling effect on the body, and it's especially popular during the hot and humid summer months. Before cooking the water spinach, rinse it thoroughly in cold running water, then leave to air-dry in a colander, with the hollow stems facing down so the water can drain out of them.
The stems are tougher than the leaves, so if you're going purely on taste, rather than presentation, it's better to cut the stalks in half and start cooking them a few seconds before adding the top portion of the plant. Water spinach looks better when cooked whole, though - so most restaurants don't bother halving the stalks.
The easiest way to cook water spinach is to blanch it until the stems are limp but not overcooked (about 30 seconds). Drain well, then top with fu yu (fermented bean curd) that's been mashed with a splash of soy sauce and rice wine (and, if you like, a little sliced fresh chilli). There are many variations of stir-fried water spinach. Most recipes start with oil and garlic: heat the oil, add the chopped garlic, then stir-fry until fragrant. Here's where the recipes start to vary: you can simply add some sliced bird's-eye chilli; or mix in mashed belacan (fermented shrimp paste) with sliced chilli; yellow bean paste (sold in jars); sambal; or just a little soy sauce. Let the aromatics sizzle a little in the oil before adding the water spinach. Stir-fry the water spinach constantly and cook until the stems start to wilt, then remove from the wok and serve.