After you've fried up a delicious batch of homemade donuts, what do you do with the leftover cooking oil? There's an old wives' tale that you can pour used cooking oil into the ground, where it becomes food for earthworms. But the reality is, used oil will likely move its way to waterways -- through storm drains or natural methods -- where it will be a contaminant. Oil and water don't mix and can poison lakes, rivers and the public water supply. Instead, try reusing the oil by cleaning it and filtering it through coffee filters. Or find a local recycler who can turn the oil into biofuel. If you need to get rid of it faster than that, mix it with kitty litter and place it in an airtight container to toss in the trash.