Wiki CAPTCHA questions

To cut down on spam from bots (very effectively so far), we’re using a CAPTCHA in the form of a question, randomly selected from a small set, on the wiki user registration page.

It would be good to get some more questions – right now there’s just four – so we’re looking for suggestions. They need to be very easy, so anyone will know the answer, and it would be nice if some at least were crypto-related. There can be more than one right answer, but not too many. It’s case-insensitive. Here are the current ones with the answers as examples:

'Which cryptocurrency is this wiki about (three letters)?' => 'ADA',
'Which is the first and best known cryptocurrency?' => ['btc', 'bitcoin'],
'How many fingers do most human hands have (counting the thumb)?' => [ 5, 'five' ],
'How many toes do most human feet have?' => [ 5, 'five' ],

So, any suggestions for more? Don’t worry about the format, I’ll fix it.

(IMO exposing questions and answers here is not a significant security risk because the site is not being individually targeted, and not likely to be. Even before the CAPTCHA spam incidents were only averaging about one a day, and since, there have been none. And the worst case scenario is some new questions are needed.)