While at first putting your greyhound in a crate may seem cruel, greyhounds can suffer from separation anxiety when you are gone. The point of crate training is to have your dog accept that he will be alone for part of the day and that you will be coming back. The crate should be large enough for the dog to stand and turn around in comfortably. Make it into as pleasant of a place as possible with padding and toys--never use the crate as punishment.