The higher the note, the further a garden eel rises out of the sand. That's your only clue.

■How to play

1. Hear the reference note. Or skip it and go in cold

2. The phrase plays. One by one, the eels rise

3. Answer the glowing eel's note on the keyboard

The stretch is relative — lowest to highest note of the phrase. Leave it alone and it sinks back into the sand, so the question is how much you still remember while the music is playing. You can hear it again, at a cost to your score.

Miss, and that eel greys out and tells you one thing: "higher" or "lower". Get it right and it beams at you. Get it wrong and it hides. Keep a streak going and the eels behind it start to dance.

■White eels, spotted eels

White garden eels take the white keys. Spotted garden eels take the black ones. The song decides who shows up — and picking a song with black keys in it means you've chosen a hard day.

■Every eel has its habits

Chinta, Genta, Kirata, Shinota, Gonta. Tempo, timbre, the way they stretch, the scoring multiplier — all of it changes with who's on stage. The ones that rise slowly and sink slowly are kind. The ones that snap back down are not.

■Collect them

Clear a stage and you earn a stamp for your field guide. The eels you haven't met stay as silhouettes, and some don't even show a slot. Meet the same eel again and again and its trust grows — bronze frame, then silver, gold, rainbow. Here, duplicates are good news.

■A song takes a few minutes

You can go meet one eel before bed and call it a night. Whether your ear is actually getting better is a question your field guide will answer for you.

■Music

Public domain classical pieces. 〔track count TBC〕