Take one can black beans. Use half, then store the other half in an opaque container in the back of the refrigerator. Wait until you have forgotten about them. Discover them, sniff cautiously, and make a pensive face. Be unsure as to their okay-to-eat status. Rinse them thoroughly, then place them in a saucepan to simmer. After a while, add some cream cheese or sour cream. Blend until you have purple goo with beany lumps. Put it in a clean container, then return it to the back of the fridge. Wait until you have forgotten about them again and they develop fur. Make a blechy face and throw the whole mess away.