Modern playpens can be moved easily. They are usually made of a basic metal and plastic support system with mesh, soft plastic or nylon sides. Sometimes a bassinet can be put on the top for the baby to sleep in, or be changed in, until four months of age. Some have mobiles, side pockets for supplies and toys, night lights, music boxes, or a CD player for the child to listen to music while sleeping or playing. Portable playpens come in different sizes, but most can be rolled or folded up for easy storage and transport.