Glass Noodles (woon sen) are made from the starch of the mung bean and water. Their almost clear quality gives them their name. Unlike other types of noodles, Glass Noodles rarely make an appearance in the types of noodle dishes eaten for lunch. They're more commonly found in hot and spicy salads, stir-fries or as fillers for clear soups (gaeng jood).
Unlike other types of noodles, glass noodles are almost always purchased dry. To prepare the noodles, you don't need to boil them like other pastas. They will quickly soften in hot water. I usually use hot water from the kettle, but hot tap water will also work well enough.
One important thing to note about glass noodles is that, while they may appear to be brittle when dry, unlike spaghetti they retain some flexibility. So, they can't easily be snapped into smaller pieces. If you do want to break up the long noodles, it's best to cut them with some kitchen shears.