Stand with the balls of your feet on raised platform (stairs, book, etc.); legs shoulder-width apart with your toes pointing forward. Lower your heels towards the floor until you feel a stretch in your calves. Keeping your knees straight but not locked, raise yourself onto the tips of your toes. Repeat.

Advanced version: do the exercise one leg at a time.

Do three sets of 12 repetitions, resting no more than 90 seconds between each set.
This exercise strengthens the calf muscles.