Japanese tuna

Chinho went to Fusha (supermarket) today, so I gave him 300 yen to buy me a can of tuna. I told him that tuna is "maguro" in Japanese which is totally different from "Mackarel". I also wanted to show him the kanji of Tuna, but Chinho rejected saying of course he knows the kanji of tuna. So he went to shop, returned and pulled this out from the bag:



Sardines!!!!
He said he thougth tuna and sardines are the same! Yes, of course they are the same. Fish-fish.
Chinho: But I think they taste the same.
Me:No, Chinho, tuna is sour and sardine is sweet.
Chinho: No, sardine is sour, too!
Me:Ok, taste it.
Chinho took a bit of the sardine, tasted it and said:
Hmmm, Japanese tuna is sweet!