さい, せい
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You need to stop a drunkard from going down a slide with a jackhammer to ask him how many years old he is. He's doing a lot of dangerous things right now, and they all have age limits.
When you ask him how many years old he is, he turns to you and screams, "I'M A MILLION YEARS OLD AND YOU'LL NEVER STOP ME!" That's obviously not how old this kid is, so you take out your holy blue saber (せい) and slice the slide in half. Then the jackhammer. Then the kid's metal arm—wait! This kid is a cyborg (さい)!
Currently there are no similar looking kanji which exist in the WaniKani learning system