Words that belong together go in the same row.

Forty words on an eight-by-five board. Every row has to end up holding five words that share one theme — and all eight rows have to be right at once.

A row only shows its theme once it is right. Everything else stays quiet — and mercury is a planet and a metal, which is the sort of thing that makes it hard.

How it works

You swap two words. That is the only move, which is why the puzzle can never get stuck: every row always holds exactly five words. There is no check button and nothing to submit — a row tells you it is right by going quiet, and the puzzle is over the moment all eight are.

The theme names are hidden. You can buy one with coins if a row will not come, and coins come from playing. Five difficulties, and the harder ones do not tell you less about the board — they tell you less about which word belongs where.