Simplest self-working trick ever

This trick needs 21 cards, which the spectator can shuffle.

Show the cards face-up to the spectator so that they can choose a card, without taking it from the deck !

Next, create 3 piles of 7 cards each by dealing theme one by one. Ask the spectator where their selected card is. Place this pile in between the 2 others.

Repeat this operation 3 times.

Finally, take the deck face-down, and deal card on the table. The selected card is the 11th from the top.