This is the 500th most frequent Turkish word.
içine
"İçine" means "into" or "inside" in English.
The word "içine" indicates a movement or placement directed into the interior of an object, here referring to the interior of 'my bag'.
Kitabımı çantamın içine koydum.
I put my book into my bag.
In this context, "içine" specifies a location inside an object, indicating where something (a surprise) is placed.
Dolabın içine bir sürpriz sakladım.
I hid a surprise inside the cupboard.
Here, "içine" is used metaphorically to suggest inclusion or infusion of something (joy) into an intangible entity (emotions).