Fool cards in Persona 2.
You need to find a demon with the "fool" trait (not awfully rare, but rules out a decent amount of demons), it needs to start talking to you (low chance at the start of the battle), then you need to respond to its questions in an alternating pattern of making it eager or angry (some questions have no answers with guaranteed results), and then finally there's a very small chance it'll ask you a special question which, if answered right, gives you 1 Fool card.
The best part is that - at least from what I've seen - nobody's even sure if this is totally correct. People say some of the conditions above don't necessarily need to be met, but nobody's certain. All anyone can agree on is that it's ridiculously rare, and barely hinted at in the game proper.