Overheard on Tarragon Street

Niklas, Esther and I were discussing who is a boy and who is a girl.

Most of her exclamations went like this:
"Mamma is a GIRL!"
"Pappa is a BOY!"
"Sylvia is a GIRL!"
"Hanna is a GIRL!"
"Benjamin is a BOY!"

Niklas decided to try to catch her off-guard by holding up a (slightly curved) cucumber and saying, "This cucumber is a _____?"

So Esther exclaimed, "The cucumber is a BANANA!"