ジョーク、、、水の化学式は？？

あまりにも幼稚なジョークだけど、発音のｔ聞き間違い、、子供のジュークだけど面白いね、、。

TEACHER: What is the chemical formula for water?

SARAH: "HIJKLMNO"!

TEACHER: What are you talking about?

SARAH: Yesterday you said its H to O!