You put them in a plastic bag so that some critter doesn't try to dig them up and eat them.
edit: Damnit, this fucking chihuahua showed up on my doorstep a few days ago, so my mom and I have been taking care of it. The owners still haven't claimed him even though we've put up posters everywhere. I normally wouldn't mind this because I like dogs, but this dog keeps on pissing and shitting in the house even though he's been getting taken out every 2-3 hours. If the owners don't claim him, then I have a bit of a dilemma on my hands. I could give him to somebody else without telling them that he will piss and shit everywhere and hope that they choose to regift him to somebody else and not give him back to me, or I could take him to the shelter. The latter option seems a bit harsh. I would pretty much be giving him to an animal death camp for not keeping his bladder under control. Then again, I'm not really sure that I care.