Lie on your back on a mat with your legs bent, holding a 5lb dumbbell in your left hand and 3lb dumbbell in your right hand. Extend your arms toward the ceiling and lift your upper body off the mat in a crunch.
Lower your back to the mat. Keeping your right arm straight, lower it to the side. At the same time, bend your left arm at the elbow to lower the weight until it's near your left ear. Do the crunch-and-lower sequence 10 times, then switch the weights to the opposite hand and do ten more reps, lowering your left arm out to the side and bending your right arm until the weight is near your right ear.