Flex eggs are special eggs in a Celestial's inventory that can be filled by a number of eligible Zapped eggs. Which eggs are eligible are indicated by: the symbol on the egg (Rare or Epic); the required Element (appears first in the displayed sigils); and the number of sigils. For example, a Flex egg with the Rare symbol, the Earth Element and 3 sigils can filled by a Rare T-Rox or a Rare Pummel (among others).