Kettlebells are perhaps the biggest trend in fitness. But if you’ve never used one, you might wonder, “What’s the big deal? They’re basically just a different kind of dumbbell.” Well, that’s what makes them special. These Russian imports—which look like bowling balls with handles—can be used like dumbbells, but you’ll find that they can make the same exercises more challenging. That’s because the weight is off-center, which forces your stabilizer muscles to work even harder.