You'd be surprised how much your kid knows, even at 13 months. If he can say 10 words, he probably understands at least 20 more. dont't try to trick him. That's not the way to earn his trust and get him to work with you in the future, cos even if you get away with it now, you won't later on. Explain it to him as best as you can, as simply as you can. Repeat it using as many words that you think he knows as possible. You'd be surprised that he would actually listen to you.