[bena]

This is one where if you have the time and effort to dedicate to it, can be simple. You memorize your deck, so when they tell you the card, you thumb to it and flip it so when you fan it, the card they picked is upside down.

[gibybo]

The "Invisible Deck" is a very famous trick. It can be purchased[1] for about $10 and just about anyone can master it in about 10 minutes. With the standard method, you don't need to memorize the deck or flip the card.

[1] https://www.amazon.com/U-S-PLAYING-CARD-COMPANY-SG_B002MI1B3...

[rkagerer]

What did you buy today honey?

A box of invisible cards.

[fortran77]

This is not how invisible deck is done though. It takes a gimmicked deck, no memorization, and not too much skill.

[bombcar]

That's close to how the version I know worked, but it was a step or two from that (and a bit simpler).