Foundation
Crabs in the dirt make a great foundation for a seaside hut. It's right on the seashore, where all the crabs dig their holes in the dirt. But as they get old and die, their bodies form a strong base. Perfect for the foundation of a building like this!
Do you live on the beach? Maybe the foundation of your house was made from crabs in dirt too. Why don't you go outside and check?
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Remember, a good foundation is the key (き) to a good house. No, literally. You have to use a piece of the foundation to make the key to your house.
Keys to houses are always made out of a piece of the foundation it was built upon. How else would they be unique from one another?