There are two objectives for captchas: 1. Prevent prefab'd spam bots from being able to spam your site. Almost anything, including trivial questions and Sony'

There are two objectives for captchas: 1. Prevent prefab'd spam bots from being able to spam your site. Almost anything, including trivial questions and Sony's "captcha", works for this. Some bots will randomly try values for unknown fields, and note if a value apparently succeeds where others failed; against such bots, questions like "2 + 2 = " or "type 'orange':" are bad. A decade ago, people were writing irc bots that could answer most trivial captcha questions. A question like "Who was the U.S. President in 1993-2001" has a better chance of going uncracked. 2. Prevent a targetted spam bot from being able to submit arbitrarily many forms on your site. "orange", "3 + 5 =", 31337e^(-pii), and sony's captcha will not work for this. If there's a question pool, someone will solve most or all of them and program the bot to respond correctly, most of the time / all the time. The idea that someone would only ever want to do (1) is flawed. If someone's bot is flexible enough, and your question pool is shallow enough, it may be worth their time to code in responses to your custom questions even if your site caters to a relatively small audience.