Sum Stack Game

Pieces of two or three digit blocks fall into a well. Move and turn them as they drop, and when a piece lands, any group of touching blocks that includes it and adds to exactly 10 lights up and clears. The blocks above fall into the gap, and if they make another 10, that clears too. Full rows do nothing here. Only the sums matter.

Steer falling digit blocks into a well so touching blocks add to 10, and set up chains as the blocks above fall in.

How to play

A piece of two or three blocks falls into the well, each block holding a digit from 1 to 9. The next piece shows beside the well.
Move and turn the piece as it falls. A ghost in the well shows where it will land.
When it lands, touching blocks, side to side or up and down, that include the new piece and add to exactly 10 light up and clear. If a landing could make more than one 10, the largest group clears first, and then every other group that shares no block with it.
Blocks above a gap fall straight down. If they now make 10 with the blocks they touch, those clear too, as the next step of a chain.
If a piece cannot enter the well at the top, the game is over.

A piece falls one row every 0.8 seconds at level 1. Every 8 groups cleared is a new level, where a row takes 0.85 times as long as before, down to 0.12 seconds.

On a keyboard, press ← and → or A and D to move, ↑, W or X to turn clockwise, Z or Q to turn the other way, ↓ or S to go down a row, and Enter to drop. On a phone, drag sideways to move, tap to turn and swipe down to drop, or use the buttons under the well. Escape or Space pauses, and pausing hides the digits, so it cannot buy thinking time.

Scoring

Each block in a group that clears scores 10 points.
In a chain, each step multiplies that by its number: 20 points a block in the second step, 30 in the third, and so on.

Your five best games are saved in this browser, with the level each one reached. In practice, the blocks the piece would clear are lit before it lands, a full well empties instead of ending the game, and you can end the game from the pause screen. Practice games are not saved.

Number bonds to 10

The pairs that make 10, such as 3 and 7 or 4 and 6, are called number bonds, and knowing them without counting is the first step in fast mental addition. Abacus users learn the same pairs as complements of ten, and a mental abacus relies on them for carrying. Groups of three or four blocks practise bonds with more than two numbers, and planning a chain means adding numbers that are not touching yet.

For more ways to train, see our mental math techniques, or the mental math apps other players use.

Try our other free memory games.