"If you give a mouse a cookie, he's going to ask for a glass of milk. When you give him the milk, he'll probably ask you for a straw. When he's finished, he'll ask you for a napkin. Then he'll want to look in a mirror to make sure he doesn't have a milk mustache. When he looks in the mirror, he might notice his hair needs a trim. So he'll probably ask for a pair of nail scissors. When he's finished giving himself a trim, he'll want a broom to sweep it up. He'll start sweeping. He might get carried away and sweep every room in the house. He may even end up washing the floors as well! When he's done, he'll probably want to take a nap. You'll have to fix up a little box for him with a blanket and a pillow. He'll crawl in, make himself comfortable and fluff the pillow a few times. He'll probably ask you to read him a story. So you'll read to him from one of your books, and he'll ask to see the pictures. When he looks at the pictures, he'll get so excited he'll want to draw one of his own. He'll ask for paper and crayons. He'll draw a picture. When the picture is finished, he'll want to sign his name with a pen. Then he'll want to hang his picture on your refrigerator. Which means he'll need Scotch tape. He'll hang up his drawing and stand back to look at it. Looking at the refrigerator will remind him that he's thirsty. So... he'll ask for a glass of milk. And chances are if he asks you for a glass of milk, he's going to want a cookie to go with it."
Get it?
Next post starts it with something like, "If you give a mouse a cookie, he's going to ask for a glass of milk."
And the next poster continues either forever or until some logical stopping point. And then a reboot.