why why why?

While I am eating mangosteen, Kelvin came to me and told me he would like to try some.

Kelvin : Mummy, what is this?
Mummy: baby, this is Mangosteen, you want to try some?
Kelvin: yes, i want.
**He took a bite and continue with his question**
Kelvin: Why no mango inside the mangosteen?
Mummy: **slapped forehead** Good question, I am not sure, you ask Ah Kong!
** Coz I don't know how to answer him so I delegate this challenge to my dad**