Pour the lime juice on the chicken and let stand while you prepare the rest of the soup.
In a medium saucepan, place the coconut milk, lemon grass, grated lime peel, galanga or ginger, and (optionally) chiles.
(The optional part is that if you don't want the whole dish to taste spicy, add the chiles later; the earlier you add them, the hotter the resulting dish.)
Bring the coconut milk to a simmer.
When the soup is simmering, add the lime-soaked chicken pieces and stir to distribute them.
Reduce the heat so the soup stays just below a boil and cook for 12 to 15 minutes, or until the chicken pieces are finished cooking.
Remove from heat and serve immediately with fresh cilantro leaves for garnish.
