Boil mung beans in 2 cups of water until tender. It should take about 20 minutes. Dissolve tapioca flour in a cup of water and add to the boiling mung beans. Stir quickly and constantly to prevent the bottom from burning. It should thicken. Add more water if it gets too thick. Add more flour if too watery. A consistency of gravy is ideal. Add sugar, bring it back up to a boil and turn off the heat