Encrypted email, why do I need their key?

I want to encrypt an email and send it to another user - it has my bank card details in it.Thuinderbird says I need their public key?
Why do I need their key if I am sending an email to them?

Is there any other way to do this?

You need the key so the other system knows how to decrypt it. Your system needs to encode the email using their key, so when the email gets there, it can be decrypted.

You can call them and just read the info over the phone. Or send them the number but divide it by 2 or something, and call them and let them know to do the reverse to get the true number.

You encrypt an e-mail so that if it is intercepted it can't be read. If you send them the key, and it is intercepted, then the mail can be read (unless you encrypt the key, and then....).

Private/Public key encryption is very clever in that even if someone else has the recipients Public key they can't decrypt the message; they need the Private key to do that. So you can encrypt the message using only the Public key, but to decrypt it you need both keys; clever.

I can't see how you could do that. How would you get the decryption key to them? I'm with hang-the-9; use a more secure transmission medium, such as a phone call.

(Although I suppose you could use encryption that requires just one key and then send that key in the post; although neither transmission medium is very secure the chances of the same person intercepting both messages is remote.)

I would give them a code first that only they would have and then send the message. I suppose the code could be intercepted?
What's the difference if I have their code, someone could intercept that and pretend to be them reading the message that I send, no?