Start boiling the water in a 2 quart pot. Peel and devein the shrimp and set them aside. Cut lemon grass into pieces, 5-6 inches long. Use the back of your knife to pound the lemon grass, just to bruise it to release the flavor. If you want, you can tie the lemon grass into a knot to make it easier to manage. Drop the lemon grass in water and let boil for 5 minutes.
Put the fish sauce and 1 lime's juice into the bottom of the bowls you will serve the soup inches Crush chili pepper and add to the bowl.
Remove the stems from the kaffir lime leaves and add the leafy part to the pot. Clean and halve the mushrooms and add them to the pot. Add the shrimp and turn off the heat. Shrimp gets too tough very quickly, and will cook even when it is just sitting in the warm broth. Scoop the shrimp and liquid into the serving bowls immediately. As soon as you add the liquid to the serving bowl, you will see that the broth becomes cloudy because of the lime juice. Add the nam prig pow. Sprinkle with cilantro and serve.
Be very careful, the peppers can be hot. Take a small sip at a time. Add more fish sauce and/or lime juice if it tastes bland. It's right if it's good for your sinus.