Jessica Placzek

Each year, little blue creatures wash up along the California coast. They’re about two and a half inches across, blue, and shaped kind of like pringles. Sometimes they’re beached by the millions. They’re called velella velella.

Every year, post offices around the country are inundated with letters to Santa Claus asking for toys, candy, puppies and more. For 103 years the post office and its volunteers have been answering these letters as part of the Letters to Santa program.

When you’re living on the street, mail can be an essential way to connect with social services and keep personal relationships going. For San Francisco's homeless, there's a general delivery window in the Tenderloin post office where no home address is needed.

All week long, we've been playing this sound, and asking you to guess what exactly it is and where exactly in the Bay Area we recorded it.