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
Cost
Free, no account, no ads

What it is

Galaxea corals are aggressive corals that attack other species and each other. Roll three dice and tap two of them: they are the sides of a rectangle, and 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.

A small reef, and the three dice just rolled above it: one, two and four. The two and the four are tapped and glow blue, three corners of the blue coral light up, and a four-by-two rectangle appears hanging on one of them, out in the water.4 × 2 = 8
Tap two dice. These will be the sides of a rectangle. Your new rectangle must share exactly one corner with one of your old rectangles. Tap on a corner that is not inside an opponent’s rectangle.
Four ways to hang a new rectangle on one corner of a blue rectangle. The two flat ones and the one lying back over the coral are struck out in red; only the fourth, growing diagonally outwards, is drawn for real.
No two edges can overlap.
A blue rectangle is drawn clean across a red one. The six squares where blue covers red 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.

Alternate turns until one player cannot place a rectangle. They shade in one of their rectangles. The other player shades in two of their rectangles. 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.

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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.

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Near it on the shelf