(we have an example input <form> here)

Guess the letters and numbers
(passphrase riddle)
'E' +3 letters
small 'D' +2 letters;
1 chars before O
1 chars before small X
2 chars before small N
→ retype that here
Enter the correct letters and numbers from the image into the text box. This small test serves as access restriction against malicious bots. Simply reload the page if this graphic is too hard to read.

This is just an example. It's function-less form, which appears over and over again. You'd typically also want to provide a more senseful error message, if captcha::test() fails.