I) To keep warm and sleep comfortably out of doors, you need protective covering underneath as well as over you. II) This is because the ground is usually much colder at night. III) The best way to keep warm is to use a sleeping bag. IV) Different types of ground obviously cause different kinds of problems for putting up a tent. V) There are many different types, but the main things to consider when you buy one are filling, material and fastenings.