A shopping trolley with one wonky wheel gets away from you in aisle three and it is not stopping. It rolls itself down the aisles and every one of them ends in a corner. One tap turns it, and the only question is when. Turn early and you are into the shelves; turn late and you sweep up whatever somebody dropped on the corner, or the sale sticker on the very tip, and the whole display of canned tomatoes behind it. A grown up is holding the handle for the first eight aisles. After that you are on your own.