Parlor game
Make a blackjack on the grid. A row, column, or diagonal that totals exactly 21 pays. Go over and that line busts.
Aces are 1 or 11. Faces are 10 — King plays again, Queen swaps, Jack cuts.
A square marked 21 is one card from a blackjack. Select a card to preview whether that square pays or busts. Undo takes a face-card power back before you finish it.