Bring to a boil enough water to cover the shrimp. Add the shrimp and reduce heat to a simmer. As soon as the shells start to turn pink, approximately 3 to 5 minutes, depending on size, start to test for doneness by removing one from the pot and cutting into the center. As soon as the center is white instead of grayish and translucent, drain the pot. Overcooking makes shrimp rubbery.