Bots take things literally as said. Computers are stupid when it comes to making conversation or trying to understand a human conversation, they don't exist like we do. And captcha system is designed to confuse bots which take things literally.
It's impossible for a bot to overcome captcha. Earlier there were prototypes which could defeat captcha but it requires millions of examples to churn and decode to efficiently create a result.
A California based AI research firm- Vicarious has came up with a system called a recursive cortical network which allows to interpret captcha with just few examples. The team at Vicarious AI used a neural with a twist called recursion which is a technique that allows to learn something new from analysing previous data. The previous data is stored and used to build new data when new captcha is solved. This reduces the learning curve of bot and show tremendous results in short time.
The system require only 5 training steps to solve Google's recaptcha with upto 67% efficiently. Over time it will take bots to solve the captcha and captcha system will be rendered useless.