Humans are not the only ones to grimace when they are in pain, scientists have found. Mice show their discomfort in the same way. Decoding animals' facial expressions may allow researchers and veterinarians to monitor spontaneous pain over long timescales. This may also aid the discovery of painkillers, because this type of pain is similar to that experienced by humans. Researchers typically detect pain in mice by eliciting specific reactions. Poking the hind paw, for example, causes a mouse to reflexively withdraw the paw; heating the tail makes it flick. But scientists are not agreed on how to measure unprovoked pain. To analyse facial expressions in mice, geneticist Jeffrey Mogil at McGill University in Montreal, Canada, and his colleagues have adapted a coding system used to measure pain in infants. The work is published today in Nature Methods. Mogil teamed up with Kenneth Craig, a psychologist who studies human pain at the University of British Columbia in Vancouver.