Their hands rest on the table like a pianist's. You slot pairs of cards between their fingers, then one final single card. The pairs come out and get split into two piles, and without seeing anything you name which pile the odd card is in.
It is the simplest thing on this page and one of the best for close quarters, because the spectator is holding everything and you are visibly doing nothing.
How to perform it
Have them put both hands on the table, fingers spread. Like a piano. That is where the name comes from and it is worth saying out loud.
Slot pairs of cards into the gaps between their fingers. Two cards per gap, saying "a pair" each time. Work through every gap you can reach.
For the last gap, use a single card, and say so. "And one left over." Say it plainly. It is not a secret and pretending it is makes it interesting to them.
Take the pairs back out, one gap at a time, splitting each pair between two piles. One card left, one card right, every time.
Add the odd card to whichever pile they choose. Their choice, genuinely.
Name the pile with the odd card. The pile with an odd number of cards is the one it went to. Count silently or just watch where it lands — the maths only matters if you look away.
Why it works
Because splitting pairs cannot make a pile odd.
Every pair contributes exactly one card to each pile, so however many pairs there are, both piles stay equal and both stay even. That is true regardless of the order, the number of pairs, or which hand you start with.
So one odd card is the only thing in the system that can break the tie.
Adding it makes exactly one pile odd, and that pile is the answer. There is nothing else to know and nothing to remember. It is the smallest possible amount of mathematics that still produces a trick.
Making it play
Let them place the odd card, and look away while they do it. Turning your back is worth more here than any handling — you can afford it, because the answer is in the count rather than in what you saw.
Do not count out loud. Counting is the method showing. Learn to see odd-versus-even at a glance in a small pile.
Repeat it and get it right again. Most tricks are weaker the second time. This one is stronger, because the second time they are watching for the moment you cheat.
More
All of these, with the ones you can deal through on screen, are on the
self-working card tricks page.