These are in and outs, a great exercise for your core. All you are going to do is balance your butt on a bench. You can start with your hands right behind you, grasping on to the bench. You are going to bring your knees into your chest, put your toes into the air. Make sure you breathe here, inhale on the way up, and exhale on the way out, keeping your core nice and tight the whole time. To make this exercise a little more difficult, you can just place your hands directly above your head.