Certainty Box is a gag. It consists of a small closed box, and a note saying that one can be certain that object X is not in the box. Typically, the object mentioned is clearly much larger than the box and cannot be contained by it. However, when the box is opened, something surprising should be revealed, which would invalidate the certainty in a humorous way.

The most straightforward way is to have a miniature of the object in the box. Or make it be a pun.

Examples:

1. We can be fairly certain that there is no hippopotamus in the box.

The box contains a miniature of hippopotamus.

2. A bus is larger than this box. We can be fairly certain it is not in the box.

The box contains a knot. (It is knot in the box :D)