greyhounds for racing resembles a hare, retired racers sometimes also mistake smaller dogs for a lure, causing them to set chase. Similarly, small animals including cats may also be the subject of prey-driven behavior by some greyhounds. Prior to adoption, agencies generally screen greyhounds for their suitability around small animals especially by way of "cat testing" in which a dog is carefully exposed to a cat to see his or her interest level. Some Greyhounds have a strong prey drive, having been bred to hunt and chase. For a few Greyhounds this urge can be difficult or impossible to overcome through training. Frequently, Greyhound adoption agencies require owners to keep them on-leash at all times, except in fully enclosed areas. A few agencies will also advise owners to keep their greyhounds muzzled around other dogs with which they are not familiar. While some owners disagree or ignore this advice, adoption groups like those linked here are generally very clear about this requirement. For the best interests of the breed in general they encourage owners to be cautious with Greyhounds because on the track they are trained to run and chase regardless of weather or injury or individual desire, while at the same time breeders select dogs with a strong prey drive for breeding. Greyhounds have very thin skin and very short fur. They can be easily harmed by biting or scratching from other dogs or accidentally running into sharp objects. Combined with their low body fat, coats or sweaters are required when outside in cold weather. Some owners also put boots on their Greyhounds in very cold temperatures. In the home, the vast majority of greyhounds are easily trained for housebreaking. Some suspect Greyhounds consider the house to be an extension of their crate, and thus will not voluntarily urinate or defecate indoors except in exigent circumstances, such as severe intestinal upset. Greyhounds enjoy resting on beds and sofas, often sleeping or dozing 16 hours a day or more. They often "dig up" their bedding materials like blankets to create a pile that can cushion their deep rib cage, which can make it difficult for them to get comfortable. Particularly common in sleeping among Greyhounds is the so-called "roach" or "cockroach" sleeping position, in which a Greyhound will roll onto his/her back and spread his/her legs at odd angles in all directions, in a position reminiscent of deceased cockroaches who