猫に鰹節 (neko ni katsuobushi) fish to a cat.
Like a fish to a cat. This proverb is used when you comparing a situation where something would want something else obviously as giving a cat a fish. When the outcome of the situation being compared is blatantly obvious such as how a cat would take a fish every time. Such situation would be leaving the door to your house unlocked leaving it open for a robber or thief to easily come in take your valuables.
Note: katsuobushi literally means Bonito Flakes. Bonito Flakes are made from dried, fermented and smoked skipjack tuna. Most of the time it is shaved into flakes and sprinkle onto of Takoyaki or Okonomiyaki.
Kei is talking to his mother about leaving cake on the table, and leaving his little brother home alone.
Kei:テーブルの上にケーキを置いたまま弟に留守番をさせるなんて、猫に鰹節というものだよ (teeburu no ue ni keeki wo oitamama otouto ni rusubann wo saseru nannte, neko ni katsuobushi to ui mono dayo.)
Mother: そうだね。ならケーキをかたずけて行こう。 (soudane. nara keeki wo katazukete ikou.)
Translation
Kei: Leaving the cake on the table with only my brother some is like putting a fish in front of a cat.
Mother: That is true. I guess we should put the cake away before we leave.
