Tap three tiles, then pass them.
Pass the three tiles your line can spare. Keep every tile the line needs for three stars.
Before a single tile is drawn, an American mahjong hand is shaped by the Charleston: three tiles to the right, three across, three to the left, and in many games a second time round. This trainer drills the only decision that matters in it: what to give away.
You are dealt thirteen tiles and shown a line: five groups, the shape of a finished American hand. Choose three tiles and pass them right; the table passes three back. Then three across, then three to the left. Some levels run a second Charleston, so the same three directions come round again.
Every pass, the trainer tells you how many of the three tiles you sent the line actually needed. Passing a tile the line wants raises your cost, because a real table will not always send it back. What returns is a mix: most passes bring a tile the line still needs, some bring junk, the same uncertainty you live with at a real table. When the passing ends, your score is the tiles on your line minus the tiles you gave away that the line wanted. That is your skill score. Three stars needs a strong skill score: twelve tiles in the first levels, eleven in the middle, ten or nine in the later tables. Two stars means a workable hand one step below.
The lines used here are generic hand shapes: consecutive pungs, runs through the suits, winds and dragons, even and odd numbers, not the National Mah Jongg League card itself, which is published each year and governs real play. Your stars and unlocked levels are remembered on this device.