A clock. It runs all the time, it “sings” for hourly chimes, cuckoos or alarms, it doesn’t have a head (but does have hands) and you look a clock in the face to tell the time.
A man wants to get into a night club where the bouncers test each person before they can enter. The man waits and watches. A woman walks up to the bouncers and the bouncers say 6. The woman replies with 3 and goes in. A small man walks up and they say 12. The man replies 6 and is admitted. The first guy thinks he’s figured it out and walks up. They say 4 and the man replies with 2, but they don’t let him in.
Zero is nothing, yet when you add zeroes to a number like 1, it becomes 10, 100 or 1,000.
The ancient Greeks did not have a name for zero and did not use a placeholder. They seemed unsure about the status of zero as a number, philosophizing about how nothing could be something. Once zero was added, it allowed for more mathematical understanding.