How do I encrypt text with openssl?

The openssl CLI tool is a bag of random tricks. One of them is the enc command. Here’s an example of encrypting and decrypting some text:

$ echo 'super secret message' > plain.txt
$ openssl enc -k secretpassword123 -aes256 -base64 -e -in plain.txt -out cipher.txt
$ cat cipher.txt
$ openssl enc -k secretpassword123 -aes256 -base64 -d -in cipher.txt -out plain_again.txt
$ cat plain_again.txt
super secret message