Taking your request literally, I came up with this first draft: Imagine you and your friends have a special clubhouse. You wouldn’t want anyone else to come in

Taking your request literally, I came up with this first draft: Imagine you and your friends have a special clubhouse. You wouldn’t want anyone else to come in, right? At first you all decide on a secret word to use that you can whisper through the closed door. This way the friends that are already in the club house know that the one standing in front of the door belongs to the group and lets them in. One day that secret word gets out and kids who shouldn’t be are in your clubhouse. After throwing out the baddies, you come up with a new secret word and make sure to tell it only to the right kids. Some time later you find more cool kids to join your clubhouse and maybe there are kids that you no longer want to be friends with. It gets hard to constantly come up with new secret words and share it with only the kids you want in your clubhouse. So you have the idea to give everyone their own secret word. This way, if one of those secret words gets out, you don’t have to make a new secret word for all your friends. Soon nobody can remember which secret word belongs to which friend. So you need to collect photos of your friends and make drawings of their secret word on the back of their pictures so everybody in the clubhouse can make sure that the right friend is using the right secret word. Everything is good for a while and then somebody breaks into the clubhouse and discovers all the secret words and how your friends look like that these words belong to. To not have your secret words revealed in such a way ever again, you come up with a clever way to hide the secret words in a drawing of something else. So for example one of the secret words is „water“ and you draw a fish. A fish lives in the water so you know that the secret word is correct. But the secret clubhouse is not very secret if anybody, who can break into it, can find out who belongs to the clubhouse by looking at all the photos. The way you solve it, is to remove all the photos and keep just the drawings making sure that each friend’s drawing shows things coming from a unique secret word. To make it really safe you decide to never have anyone say their secret word out loud. The way you do this is to always use a thing that comes from the secret word. So when their secret word is “water” you let that friend say “dolphin” or “octopus”. One of the drawings you have in the clubhouse still shows a fish that also lives in the water. This way you know you can let them in.