An empty cup is an item that is obtained after drinking a cup of tea or a cup of nettle tea. To obtain an empty cup, players can buy or steal a cup of tea from the tea stall in east Varrock. There is also another way by talking to Galahad, and just having a conversation with him and he will end up giving you one, and then you just have to drink it for an empty cup.
The only use for an empty cup is to fill it up with standard tea, nettle tea or turn it into a Guthix rest potion after completion of the One Small Favour quest. To refill the cup with standard tea, players must use it on a tea flask with at least one dose in it. To refill the cup with nettle tea, players must use it on a bowl of nettle tea. To create a Guthix rest potion, a player must add a bowl of hot water to the cup and then add a harralander, a marrentill and 2 guam leaves.
Players can purchase empty cups from the Chest in the basement of Lumbridge Castle, after completion of the Recipe for Disaster sub-quests the stock will increase. This is the fastest method of obtaining empty cups on an ironman account.