Galaxea

Two of the three dice are the sides of a rectangle — and the game is which two, and where to hang it.

The Galaxea board

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Grades
3–6
Subjects
Shapes & Space, Multiplying & Dividing, Adding & Subtracting
Kind
Two-player game

What it is

Galaxea corals are aggressive corals that attack other species and each other. Roll three dice and choose two of them as the sides of a rectangle. It must share exactly one corner with one of your old rectangles. No two edges can overlap. Open water scores nothing — attack the other coral and you gain the area of overlap. When one player cannot place a rectangle the game ends: they shade in one of their rectangles, the other player shades in two, and each score is the shaded area.

The rules

Galaxea corals are aggressive corals that attack other species and each other.

Three dice roll in above a small reef: one, two and four. The two and the four are tapped and glow blue, and a blue rectangle four long and two high appears on the reef.4 × 2 = 8
Tap two dice. These will be the sides of a rectangle.
Further on in the game. One corner of the blue coral glows. A rectangle tried on another blue corner, one that lies inside a brown rectangle, is struck out in red; the new blue rectangle appears on the glowing corner, touching the old one at that corner only.
Your new rectangle must share exactly one corner with one of your old rectangles. That starting corner cannot be inside an opponent’s rectangle.
A blue rectangle is drawn clean across a brown one. The six squares where blue covers brown are shaded blue and counted: water covered once scores nothing — the squares covered twice are the score.6
You can attack an opponent’s rectangle. Gain points equal to the area of overlap.
Partway through a game, a blue rectangle is tried on a blue corner, just under a brown rectangle. Its top edge would run along the brown one’s bottom edge, so that stretch lights up and the rectangle is struck out.
No two edges can overlap.
The end of a game: brown cannot place a rectangle. A blue rectangle that brown has grown over is shaded in and struck out, because it is not empty. Then two empty blue rectangles are shaded blue and one empty brown rectangle is shaded brown.
Alternate turns until one player cannot place a rectangle. They shade in one of their empty rectangles. The other player shades in two of their empty rectangles.
Two scores: blue 36, brown 41. A sign appears between them, and the bigger score fills with its colour.364141<
Each player’s score equals their shaded area.

What it gives you to practise

Area as multiplication, chosen rather than computed. The arithmetic is in deciding which two of three dice to spend and where to hang what they make — and the third die, the one you did not use, is part of that decision. Edges may cross but may never overlap, which quietly limits where a corner can go.

In a classroom

Gordon Hamilton, MathPickle. Three rivals to play on three reefs — one that always draws long, skinny rectangles, one that grabs, one that plans ahead — a hot seat for two on one tablet, and an online room. It was the first two-player game we published, and it is still the quickest of them to explain.

Share or embed

The whole game at one address, with every board in it. Whoever opens it stays inside it, and their progress stays in their own browser.

Play Galaxea

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