One of the best things about money is that it is fungible, meaning it is mutually interchangeable. When trying to save money this fungibility is an issue. We devise ways to make money less fungible in our minds, using what is called “mental accounting”. This might mean categorising some of the money as part of a “Christmas Fund”, not to be used for any other purpose. Some people even go as far as to make some of their money harder to access until they need it. Piggy banks are one such way, but there are more scalable methods.