Try this on one of your "smarter" friends.
Use the spot-cards from Ace to Six and arrange them on the table like this:
. . .
You and your victim will take turns removing cards and keep a running total.
The first to reach (without going over) wins the game.
If you play first, take a .
Whatever he takes, take the "7-complement".
. . If he takes a 5, take a 2. If he takes an Ace, take a 6.
The totals (after your turn) will be:
. . And after his next move, you will reach 31 with your next move.
If he plays first, try make the total 3 or 10 as soon as possible.
. . And proceed as described above.
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
So far, this game is not very impressive, is it?
Here's where the swindle comes in.
Eventually he will catch on that you always start with a 3
. . and he may discern the rule of the 7-complement.
(Better yet, have a "shill" take him aside and explain the winning strategy.)
Confident that he will win, he may even raise the stakes
. . (not that I encourage gambling, mind you).
He begins by taking a 3.
You take a 3. .He will take a 4. .Total: 10
You take another 3. .He takes another 4. .Total: 17
You take a 4. .He takes a 3. .Total: 24
You take a 4.
He will reach for a 3 and find to his horror that there are no more 3's!
He must either go over 31 and lose . . . or go under 31 and allow you to win.