Are you a robot?

Have you ever been on one of those websites that asks if you are a robot? Yeah, I know you have, they are everywhere. What you might be wondering is why a computer is asking you to prove your humanity and why a robot wouldn't just lie its way in. After all, it's not like robots are sworn to an oath of candor. Also, there are videos on the internet with robots clicking the "I'm not a robot" checkbox, so what exactly do these seek to accomplish apart from just being annoying.