Any Web user is a potential knowledge contributor, but it remains a challenge to make them devote their time contributing to some purpose. In order to align individual with social interests, we selected the CAPTCHA Web resource protection application to embed knowledge elicitation within the users' main task of accessing a Web resource. Consequently, unlike previous knowledge acquisition approaches, no extra effort is expected from users since they are already willing to use a CAPTCHA to perform some particular task. We present an application where we extract pictorial knowledge from Web users, and experiments suggest that our approach enables knowledge acquisition while still satisfying CAPTCHA's security requirements.