As she gets ready to crack the last two eggs for the cake why does she refuse to continue?

When Nacha reaches to crack the final egg Tita stops her and grabs the egg claiming that she can hear a baby chick peeping inside. Mama Elena, overhearing the commotion, chastises her daughter for her behavior and cracks the egg into the bowl. It shocks Tita to see that there is no chick inside but only a very fresh egg.