Select a password and see how many tries it will take to guess it. You can only used certain characters in your password (though you can change which ones you are allowed in the program). And you need to set a maximum length for the password. The number of guesses required gets pretty huge pretty quickly. We tried putting in all the letters of the alphabet, but Scratch, even though it's astonishingly faster than any human, is too slow for it to be any fun.
This project was undertaken after a chat with J and N (10 and 8 at the time) about passwords. School had told them to use punctuation marks and mis-spelled words. And I was explaining that something that made a big difference was making the password longer. We wrote this code together to get a feel for how many guesses it takes to crack a password by 'brute force'. The algorithm could be optimised, e.g. to guess shorter passwords first. It doesn't try normally spelled words first or anything like that. And it illustrates that lots of guesses are needed. If we expand the range of valid characters, the number of guesses needed for a given password (typically) goes up.