背比べ

せいくらべ【背比べ】

¶～をしてみよう

Let's see which of us is taller.

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せいくらべ【背比べ】

seeing who's [which of 《us》 is] taller.

～する see which 《of us》 is taller; measure oneself against sb.

●ちょっと背比べをしてみよう. Let's see who's [which is] taller.