The Chocolate Thief Trap

There is a box of chocolates in your kitchen. Or — there was. On Tuesday you counted eleven. You always count them; this turns out to be important. Wednesday morning: nine. Thursday: six. Friday: two. This is suspicious. You have asked everyone. Your father hasn't touched them. Your mother hasn't touched them. The dog cannot reach the counter, and even if she could, she has no hands. Your sibling says they haven't touched them either — which you know is a lie, because they said it with their mouth full of something. You need proof. But you cannot watch the box all day: you have school. And you cannot post the dog as a guard, because the dog is a traitor who would let anyone steal anything for one biscuit. So you need a box that watches itself — a box that, the moment it is opened, makes a tremendous amount of noise, in front of everyone. Today, your team is going to build one.