By now, you probably know that exercise is a great mood-lifter. But if the idea of running a marathon or having a power session at the gym isn't exactly your cup of tea, don't sweat it. As it turns out, even the easiest exercise can still make you a little happier. According to a study published in PLOS One, small amounts of exercise can improve your mood in a big way. For the study, researchers tracked 10,000 participants who downloaded an app from the Google app store onto their Androids. Along with tracking the participants' exercise, the app also asked them to rate their level of happiness at any given moment. After several weeks of use, the app also started asking users if they had been sitting, walking running, standing, or anything else within the last 15 minutes. It also tracked their movement and exercise (though, of course, only if they actually had their phones on them while working out). The result? People reported feeling happier if they had done some sort of activity within those 15 minutes. And for the most part, people weren't doing any intense workouts, just some mild walking. As the study shows, even small amounts of light exercise can improve your mood. And equally importantly, the study further solidifies that exercise isn't just about being physically healthy — it's about being mentally healthy, too.