Summarize this article in one sentence.
A warm up gradually increases your heart rate, body temperature, and blood flow. Warming up your body before exercise can help to prevent injury and reduce soreness from a workout.  A warm up should be a light version of your planned exercise. Warm ups should last for around ten minutes. You shouldn't feel exhausted or overly tired from your warm up. For example, before going for a fast walk, warm up by walking slowly for around ten minutes. It's important to gradually cool your body down after a workout. Cooling down can help your heart rate return to a normal resting rate and may help prevent injury or soreness after the workout.  A cool down requires you to gradually reduce the intensity of your chosen exercise or activity. Try to cool down for around ten minutes. You may want to include stretching in your cool-down. For example, if you went for a walk, reduce your pace for around ten minutes. Include stretching in your warm-ups and cool-downs to help your muscles stay flexible and maintain their full range of motion. Stretches should be held for around thirty seconds each before repeating the stretch on the opposite side of the body. Try including some of the following stretches in your workouts:  Keep your shoulders flexible by bringing your arm across your body and holding it in place with your opposite hand. Push slightly with your opposing hand and feel the stretch across your shoulder. Stretch your hamstrings by first laying down on your back. Put one of your legs up and rest your foot against the outer corner of a wall. Slowly straighten your leg and hold for thirty seconds. Repeat the same motion with your other leg. To stretch your quadriceps, grab your ankle while standing and pull it up and back. You will feel the stretch in the front of your leg. You may want to hold onto something in order to keep your balance.

Summary:
Always warm up before any exercise. Cool down after your exercise. Stretch before and after a workout.