In one sentence, describe what the following article is about:

Your core is the area of muscles in the trunk of your body. The core stabilizes your body, and a strong core allows you to perform many other exercises much more effectively. To determine your core strength: Lay on the floor on your stomach. Place a long pipe, dowel, or ruler along your back. Place hour hands on the floor, shoulder-width apart. Do a push up with your legs straightened. If your chest and stomach leave the ground at the same time, then you have a stronger core. The plank is one of the fundamental core-building activities, working many muscles in your trunk at the same time. To do the plank: Lie on the floor on your stomach and place your forearms on the ground shoulder-width apart. Push up with your arms into a push-up position, keeping your elbows and forearms resting on the floor. Keep your body straight and tighten your core muscles to help alleviate strain on your arms. Breathe evenly and hold for 60 seconds. Relax for 60 seconds and then repeat 1-3 times every day. Once you've mastered the plank, it may become too easy for you. Try a more challenging version of the plank with the side plank. To do a side plank, start in the regular plank position. Then place one forearm on the ground with your elbow directly under your shoulder. Stack your feet on top of each other, twisting your body so that one side is on the ground. Push your body up while keeping your body straight. Leg lifts will help strengthen your lower back and abdominal muscles. To do a regular leg lift, lie on your back with your arms to your sides. Alternating legs, lift one leg about 12 inches straight off the ground, slowly and steadily. Hold for about 10 seconds and then lower it slowly to the floor. Do not bend your knee. Do 10 repetitions of these for each leg. Repeat 1-3 times every day. To do a more challenging version, try lifting both legs off the ground at the same time. Raise them slowly and steadily until they are about 12 inches off the ground. Hold them steady for a count of ten and then slowly lower them to the floor. Stand tall with your feet together. Put your hands on your hips. Slowly and steadily, lift one leg straight out to the side until it is about 6 inches off the floor. Lower your leg slowly. Do not let your body lean to one side. Keep your body straight up and down. Repeat 10 times on this side. Do the same exercise for the other leg. Repeat 1-3 times every day. Tighten your core muscles and gluteus muscles (buttocks) to keep your body stable. As you get stronger, lift your leg up higher. Alternately, try lifting your leg straight out in front of you. Work to be able to lift your leg up a little higher every day. Be sure to hold onto something for balance support. A kettleball is a small iron weight with a handle. By lifting a kettleball, you are forced to use your core to maintain your balance. To strengthen your core, start with a 10-pound kettleball and work through various exercises, such as the half-kneeling halo. Half-kneeling halo: Kneel on one leg. Hold the kettleball in both hands in front of your chest. Lift the weight up to your right shoulder. Next, bring the weight around to the back of your head. Third, bring the weight to your left shoulder. Finally, bring the weight back to your chest. Repeat going in the opposite direction. Complete this set 5 times and then kneel on your other leg and do another 5 sets. Do kettleball exercises 3-4 times each week.
Evaluate your core strength. Do plank exercises. Do side plank exercises. Do leg lifts. Do side leg lifts. Train using a kettleball.