system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do animals in eggs know when to hatch?

The egg doesn't just dissolve, they actively break out. But why? Is it just their size in relation to the egg? Hormones? Something external to the egg?
gpt
Genetic programming.

Remember how you latched onto your mom's boob for your first meal? OF COURSE NOT. But you did, because it was genetically programmed that you had an instinctual latching-on process to get that milk into you (or, alternately, from a bottle if you weren't breast fed).

Animals are the same. They bust out of the egg when their genetics tells them it's time to based on their level of development, whether or not they have a yolk sac still attached (like many fish do) or they're pretty much 100% complete and ready to go.