# Nurture
The process of upgrading Pettos are focusing on **evolving** and **up-levelling star system**.
### Evolving
Although Petto's life is arranged with specific element, role and rarity in advance, the breeding leads them to become stronger and better surviver in Gloryworld.
Feeding Pettos with superfoods and materials, Pettos can grow bigger and transform into a new outlook as the same time increase their attributes.
### Level up Star system
Upgrading the star from one-star Petto to two-star Petto within the same rarity can multiply one specific Petto's attribute (randomly, maybe +15% attack or +30% defense) at a noticeable rate.
The process is combining two Pettos with the same rarity and same star level to get one star up.
The system of up-levelling star system is similar as binomial tree, illustrated as an example below.
> ***Question:** How many 2-star Legendary Dragons needed to upgrade to a 5-star Legendary Dragon?"*
>
> **Answer:** $$2^{(5-2)} = 2^3 = 8$$ ; therefore, 8 two-star Lengendary Dragons needed
The general formula is $$2^{(n - m)}$$ where:
* n is the expected star level
* m is the current star level
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