They have a flat bottom surface that makes it easier for them to stand while you fill them. (geekwrapped.com)
Play yard fabric: 100 % polyester Top surface of mattress cover: 50 % polyester, 50 % polyurethane Bottom surface of mattress cover: 100 % polyester Mattress: 100 % polyurethane foam (approved for baby products in accordance with Oeko - Tex ® Standard 100 Class I) (babybjorn.ca)
Though this is the first time scientists have seen this behavior in bowheads, other arctic whales — such as belugas — have been seen grooming themselves along abrasive bottom surfaces in Hudson Bay estuaries. (sciencemag.org)