monkey trap

Meanings

1. A cage containing a banana with a hole large enough for a monkey's hand to fit in, but not large enough for a monkey's fist (clutching a banana) to come out; anecdotally used to catch monkeys that lack the intellect to let go of the banana and run away.
2. A clever trap of any sort, that owes its success to the ineptitude or gullibility of the victim.