The clues are all about items that can either hold things or be filled. In the case of the shirts, they are full when he wears each of them. The toilet paper roll holds paper on the outside, while the cup holds drink on the inside. Some of the clues are empty- such as the shirt in the closet, the toilet paper roll, and the empty paper lunch bag- are not currently holding anything, and are empty like the bowl that is empty. The other three-- the shirt he is wearing, the cup he is drinking tea from, and the socks he is wearing, are full like the bowl of chips that is full. So if they pick the items that are full, they get the full bowl. While if they pick the clues that are empty, they get the empty bowl. At least, that is what I am thinking.