"See those two cards? They 're stuck together. Please inspect them."

»They're taped together and one is quite worked up...«

"...and they're facing outwards, exactly! You're a good observer. Being that, you will also notice that I put some money down on the table. it's yours if you can make them face each other without damaging them."

»Sounds too good to be true. Where's the catch.«

"Observant and smart! I like it! The catch is, that you have to spend that money on drinks for the two of us and if you don't succeed, you owe me the same amount of money money and I buy you a drink."

»I've had worse deals, I'll take the bet.«

Step 1: Popping a Fresh Blade

No magic, no sleight of hand, just an entirely fair and solvable puzzle that revolves around the topology of a card.

Grab a pair of cards - playing cards, toy cards, collector cards, any cards. They don't even need to be in mint condition. Just make sure no one will mourn their loss.

Cutting through them will be easier if you first dab a tiny bit of superglue in the middle and press the pair together. This way they won't move on you. If your cards have an asymmetrical back side, make sure to align them accordingly. While that is not ultra critical, aligning the edges is of great importance.

The two most common are standard poker and bridge decks. I used promotional bridge sized cards (3.5 x 2.25" as opposed to the standard playing card at 3.5 x 2.5"), so I cut 13 and 25 mm from the outside edge (mixing units, I know, light the torches and sharpen the pitchforks). If I were using full size cards, I would cut 15 instead of 13 mm from outer edge, the other dimension remains the same. The dimensions are not critical. You can substitute the 13 mm dimension with anything smaller than 1/4 of card width and 25 mm can be substituted by anything smaller than 1/3 of card length.

Note that you should never attempt to cut through material in a single pass. Start by marking out corners of the cutout by stabbing them and lightly score the edges. Once you've made a groove, you can freehand cut the cards all the way to Australia.

Step 2: Tape and Fold

When you're through, glue the pair with a piece of tape on one side only. Don't use a green electrical tape like I have unless you're trying to show someone how to make the portal cards by the means of photos and lousy jokes.

Fold one card along every cut line but gravitate slightly more towards extending the fold so you get some gaps as seen on second photo (I've put a piece of red plastic behind to make the gaps more pronounced). This will make the puzzle easier to manipulate. Exercising the joints thoroughly will help as well.

Once you flip the card around (the puzzle part of this instructable), you will have an easy access to the other side of the joint. You should align the cards again, since they are likely to move, and tape them together once again.

I would encourage you to try to solve the puzzle before moving to the solution and I would ask my friends and family to skip the next step entirely, thank you.

Step 3: Not for Friends and Family!

Tried solving it yourself? No? Really? Jumping straight to solution? Come on, you can do better. See you at next step in five or so minutes.

Also note that the photo above is not what you want to achieve (but is what you likely will end up with anyway...)

Step 4: Still Not for Friends and Family!

Did you solve it? Doesn't matter, here is the solution:

fold the side at the tape joint inwards.
fold the narrow side flaps inwards
fold the remaining flap inwards
flip the card along the long edge
unfold each flap from top to bottom
profit

Common mistake happens in step 5 where unfolding happen sort of in reverse. This mistake will make the cards face the same direction. Hard to describe with words but if you follow the photos, you should's stray too far off course.

Step 5: I'll Have an Ale, You?

I would like to open up the conclusion by apologising to my friends and family, who have endured my and brutal human trials. The conclusion is that this puzzle is not only tough to solve on your own but even when the solution is (quickly, sorry:) shown to you.

Now let's make a bet. Make this puzzle card pair and go pester the first human you come across. If they solve it without help, I'll send you a photo of a cute kitten but if they don't, you owe me a like and a vote (I'm entering this in the puzzle challenge). Deal? I can send a photo of a puppy if you prefer.

Also, credit where credit is due: this puzzle is a twist on Terry Roger's "Star gate" trick from 1985.