Rats can’t talk, but researchers say there is one easy way to tell if they’re happy: look at their ears.

A group of Swiss scientists tickled rats to make them happy for science, in a delightful study published recently in the journal PLoS One. The researchers took photos of the happy rats and compared them to photos of the same rats when they were forced to listen to unpleasant noise. (Poor things!) And just as we smile and frown, happy rats do look a little different from miserable ones. It’s subtle, but the ears of happy rats are just a little more pinkish than those of the unhappy ones. Plus, they’re angled instead of sticking straight up.

A study from earlier this year confirmed that rats do love being tickled. They make cheeping laugh sounds, jump for joy, and come back for more. You can even figure out how much they like the tickling by measuring how loudly they make their cheeping laughs. Of course, just like humans, some rats love being tickled more than others. The first part of the experiment was selecting the lucky rats that got to be tickled. The scientists spent time tickling 75 different rats. From these, the 15 rats that loved tickling were selected to be part of an elite group that went on to the next step.

Let’s get one thing straight. It wasn’t just any random tickling. There were scientific procedures. There was a one-handed tickling procedure, where you basically tickle the stomach, and a two-handed tickling procedure, where you hold the rat and tickle the sides and the nape of the neck. One-handed tickling is more common in scientific studies, but the rats seemed to like two-handed tickling more, so the researchers tried a mixture.

After selecting the 15 most tickle-loving rats, the scientists tickled these particular rats even more, and then took photos of how they looked. Next, they took the rats to a different room, made them listen to white noise that they don’t like, and photographed their unhappy expressions, too. The comparison of the photos showed that the main difference was ear color and position.

Being able to tell how rats are feeling is useful for people who care about animal welfare. The researchers suggest that there should be further studies on ear color, which is good because it means more happy rats being tickled.