“バナナはおやつに入るん”是什么意思？

香蕉可以算是零食

点心里将会有香蕉。

把香蕉放入点心里。

把香蕉归类到零食一类里去。

In primary school, Japanese student always like to go 远足(ensoku), because not only going 远足 itself but also eating 弁当（bentou, 便当）out of the room with their friends are fun.

To add to that, the fact excite children that they are allowed to bring some お菓子(okashi, snacks & sweets, 点心/糖果) as おやつ(お菓子 ate in the afternoon / お菓子 after lunch) in 远足 day, it's not allowed on ordinary school days.
All children try to buy & bring as much as they can, but the limit is 300 yen or like. (the exact number of the limit is depend on the school)
They should think hard, which お菓子 they take and give up to bring which ones.

Then, some children invent the "NEW" (actually, it's very old) way for them to bring more お菓子 than the limit.
First, they ask their mother to add some banana (or other fruits) to their 弁当, and 2nd, they buy other snacks until the limit.
They already suppos