We all know implicitly what things are hard for computers--we experience those difficulties every day. Try to tell your computer to look through your e-mails and find out if you won a million dollars. For one, there is no way to "tell" the computer that you want this. Second, even if a computer is smart or has an application that can search your e-mail, it would do so and most likely say "no." In contrast, ask a human friend or an assistant to do the same, and he would laugh at you. What even a smart computer does not know is that any e-mail announcing a million-dollar windfall is probably a hoax.

Identifying what is hard for computers and easy for humans is important for a surprising combination of conflicting reasons: We want computers to be better so that they can help us, but we also want computers to be limited so that we are able to differentiate them from humans.