A baby playpen is a piece of furniture designed to create a safe space for infants or young toddlers to play in when their parent or guardian is occupied or away. Playpens are traditionally made of wood and consist of a flat rectangular platform, usually square in shape, with vertical bars on four sides, so that the child can see out. The floor of the playpen is usually a soft mat, and the walls of the playpen are usually higher than the height of the child, so as to avoid climbing injuries. Modern playpens are portable and typically consist of a basic metal and plastic support system and mesh, soft plastic, or nylon sides. They are easy to fold up and put away and have walls that are high enough so that the child cannot climb over the top. Playpens are not made for sleeping and should only be used for playing. For safety, parents should never place soft bedding, pillows, or a mattress in the playpen, use string to tie toys to the sides of the playpen, or leave a baby in a mesh playpen with the side lowered.