Some researchers in social robotics aim to build ‘face robots’—machines that interact with human beings (or other robots) by means of facial expression and gesture. They aim, in part, to use these robots to test hypotheses concerning human social and psychological development (and disorders such as autism) in controlled, repeatable experiments. A robot may be said to ‘grin’ and ‘frown’, or to have ‘a smile on its face’. This is not to claim merely that the robot has a certain physical configuration or behaviour; nor is it to say merely that the robot’s ‘facial’ display is, like an emoticon or photograph, a representation of a smile or frown. Although researchers may refrain from claiming that their machines have emotions, they attribute expressive behaviours to them literally and without qualification. Wittgenstein said, however, ‘A smiling mouth smiles only in a human face’. Smiling is a complex conventional gesture. A facial display is a smile only if it has a certain meaning—the meaning that distinguishes a smile from a human grimace or facial tic, and from a chimpanzee’s bared-teeth display. In this paper I explore the implications of Wittgenstein’s remarks on expression for the claim that face robots can smile or frown.