Here is an example of a "Trapdoor card". It is also known as the "Spade card", and many other names besides. Like the Hypercard, it is made from a single card (in this case a playing card) which has been slit and folded - no joins, no glue or adhesive of any kind.

There is a limit to how much a photograph can convey about this bizarre creation. Essentially, it is clear that a square (with its centre missing) has been cut out of the upper half of the card, and folded downwards. However, the stem or strut which connects this square to the rest of the card is on the "wrong" side. Any attempt to simply unfold the square back to its original position will fail for this very reason. Which leaves the question, how was it made? (Thanks to Tim Rowett for this particular example.)