Make sure your pen is tall enough for your dog, especially if you have a climber. Some playpens have detachable tops made from netting or other materials to keep your pup from making a break for it. You can also make your own top by attaching a bed sheet or some garden mesh to the top. If your pen has a door, always open it and take your pup out that way. Once dogs figure out they can go over the top, they can realize climbing is one option to get out.