A transformable building block comprises a body configured to receive a connector post or other portion of another building element, an appendage movably coupled to the body, and a post configured to protrude from the body and retract into the body. Inserting a connector post into the body of the transformable building block may cause the appendage to move and the post to protrude from the body. Removing the connector post from the body may cause the post to retract into the body.