I just thought of one that they did at our school a few years ago.
They got pigs (don't ask me where or who came up with this idea) and released them in the school. The brilliant part was they named them 1, 2, and 4. Well the captured 3 pigs, 1, 2, and 4, but just could not find pig 3. LOL. They spent a LONG time looking for that darn pig 3. LOL.