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An alternative way to think about it would be a padlock and a key. Anyone can close the padlock (assuming the common spring loaded design), but only the key holder can open it.

While this works for encryption, it doesn't really work for explaining signatures or however. Maybe someone can come up with a good analogy for that case.

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That's the analogy I was taught in college. The public key is really a padlock that had infinite number of copies, but there was only one key that could open all the padlocks. If someone wanted to send a secret message to the person who owned the key, the would obtain a copy of the padlock, put their message in a box and lock it with the padlock, then send the box to the key owner. The key owner was the only person who could unlock the box and read the message.

Certificate and signature could work as an analogy for public keys, if they weren't already taken by higher level concepts.

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