There re several Captcha engines available with technologies such as solving an equation, copying a character string or the one invented by Google, checking a checkbox.

On the question and answer site, Quora, recently someone asked, “Why can’t bots check ‘I am not a robot’ checkboxes?”

You have seen those boxes. We all have checked them from time to time. On Quora, someone took the time to give a detailed answer to the question, and the extent to which Google goes to assure that robots don’t sneak through is amazing.

It’s worth 5 minutes to understand how hard it is to fool Google.

By the way, if you answer questions like this on Quora, it can provide a backlink to your website.

Why can’t bots check “I am not a robot” checkboxes?.

Quora