A public key is an address that anyone can use to send money to you while you use your private key to send money to others. The only one who knows the private key should be the owner of the private key, otherwise, other people can use the account to send money to others. To have a clearer picture of this, imagine a vending machine that anyone can put money in, but cannot take money out of because they don't have a key. The only person who can take money out is the owner of the vending machine. In this situation, the vending machine acts as a public key where people can send money to, while the machine owner has the private key that allows access to the money.
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