The problem is that we're quickly running out of things that: 1. can be done by humans most of the time 2. cannot be done by machines most of the time 3. fit

The problem is that we're quickly running out of things that: 1. can be done by humans most of the time 2. cannot be done by machines most of the time 3. fit into the workflow of a web form Point 3 is the interesting bit: You cannot build a captcha that tests dexterity, emotional intelligence or creative problem solving but is still accessible to a large internet audience.