Tripe is the lining of an animals stomach, and most of the tripe that is sold is beef tripe, which comes from cows. It is tough and needs to be cooked for a long time before it gets tender. Tripe is common in pho, a Vietnamese noodle soup, and it is prepared by washing it thoroughly before cooking. To wash tripe, cut away all the bad stuff, including fat and any section that isnt a pale, off-white color. Rub rock salt all over the tripe and rinse the salt off with vinegar. Repeat this process a couple of times, or at least until there are no impurities left. Once the tripe is clean, it can be boiled for five to ten minutes, sliced into thin strips, and added to the pho. Tripe has little intrinsic flavor and tends to take on whatever seasoning its given, but it has texture and some portions have a gelatinous substance reminiscent of calves feet that can be very soothing.