There are a few things that keep soup hot on a cold day. One is to keep it in a thermos. This will help to maintain the heat of the soup and prevent it from cooling off too quickly. Another way to keep soup hot is to put it in a slow cooker. This can help to keep the soup warm for an extended period of time. Finally, another way to keep soup hot is to put it in a pot on the stove and turn up the heat. This will help to keep the soup warm and prevent it from cooling down too quickly.