Cetris

Goal: form groups of 3 or more like-colored blocks.

How: one random block from the lower tier of the stock (the double row of 7) will drop straight down after your each move; this you can't control. But you do control the item on your tray. When that item is a colored block, drop it where it's likely to meet blocks of the same color. When the item on your tray is a black spot, that's an eraser: drop it onto a block you don't want, like a block that interferes with color grouping.

Operation: Use left and right arrow to move the tray. Hold CTRL key with down, left or right arrow to drop the tray item straight down or to either side. The tray item is then replaced with the leftmost of the two shown atop the stock.

Scoring: a group of 3 earns 1 point, 4 earns 2 points, 5 earns 4 and 6 earns 8. After a group is scored, those blocks vanish and any above it slide into the space created. The game ends when any stack reaches the top.

Timer: every 40 seconds the difficulty increases slightly, eraser spots supplied ever less frequently.