Google has basically your entire browsing history, and search history. Google then compares that with what is considered normal for a human, and then lets you in if you pass that check.

Of course, if you run IRC bots that scrape Google with a headless browser to implement a .search functionality, and which offer link titling in IRC, and you use a separate bucket of cookies and IP for every IRC bot, your bots now also have a human search and browsing history, and also will pass all ReCaptchas...