A rule-switching game. A letter-number card lands in the top or bottom half of the board; the top half asks whether the letter is a vowel, the bottom half asks whether the number is even. You answer yes or no, and one wrong answer ends the streak.

Why is the position the whole game?

Because the same card gives opposite answers depending on where it sits. The skill being trained is switching which rule you apply on an external cue · not spelling or arithmetic, which stay deliberately trivial.

Which letters count as vowels?

A, E, I, O and U. Y never appears, precisely because languages disagree about it.

Yes. At lower difficulties each zone shows its rule as a caption; from difficulty six the captions vanish and both rules live in your working memory.

How does my progress save?

Your best streak is stored locally in your browser. Sign in to sync it across devices.

Rule Flip · 母音か偶数か · 日本語

One card, two rules · top half asks about the letter, bottom half asks about the number. Apply the right one

Top half asks if the letter is a vowel; bottom half asks if the number is even. JavaScript required.

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