Serial Comma (Within List Of Similar Elements)
When creating a list or series of multiple things which are similar, commas should be used to separate each item in the list.
N.B. American English requires the use of a comma before the last and in a list; British English does not. Be sure to follow local protocol, particularly in formal writing.
Teenagers are often anxious to grow up, get a job, and move out of their parents’ house.
If you look carefully, there are three things teenagers want to do: 1) grow up, 2) get a job, and 3) move out of their parents’ house. These are similar things (they’re all things teenagers are anxious to do), so we need both commas.
I still have to buy a gift, pack the suitcases, and arrange for someone to water the plants while we’re at the wedding.
Mary needs bread, milk, and butter at the grocery store.