月と鼈
(Tsuki to suppon; “The moon and a turtle shell”)

Definition:

The moon is round, and a turtle shell is also round. But they have essentially nothing else in common. This kotowaza refers to things that, while they may have some superficial similarity, are in effect so different as to make any attempt at comparison meaningless.

Breakdown:

Two nouns and a particle! 月 (tsuki) is “moon” or “month”; 鼈 (suppon) is the Chinese (or Asiatic) softshell turtle, a freshwater snapping turtle native to east Asia. The particle と (to, pronounced like “toe”) here is a closed “and.” That’s all there is to it.

Notes:

Apparently a native English equivalent would be “like chalk and cheese.” Other Japanese phrases of similar meaning include 雲泥の差 (un-dei no sa), “as different as clouds and mud,” and the more closely-related 提灯に釣鐘 (chouchin ni tsurigane), “paper lanterns and hanging bells,” which also lists two things that may appear similar at first glance but are really very different.

Example sentence:

「さすが先輩、上手ですね。プロみたい！」　「いやいや、俺とプロを比べたら、月とすっぽんだよ」

(“Sasuga sempai, jouzu desu ne. Puro mitai!” “Iya iya, ore to puro wo kurabetara, tsuki no suppon da yo.”)

[“Wow, you’re really good. You’re like a pro!” “No, no, comparing me to a pro is like comparing a turtle to the moon.”]