When you take part in food or drink at the home of a person with magical powers, you might be put under obligation to that person. Obligations is services you have to do for the host. A way to circumvent this is to ask the host if accepting food or drink from them puts one under obligation to them.[1]
After a cup of tea and a biscuit in 1997, "Uncle Bailiff" is under obligation to Mama Thames, and works as a sort of handy man for her.[2]