C.S. Lewis, in "Mere Christianity," uses the metaphor of an egg to illustrate the struggle of personal growth and transformation. He suggests that while it may be difficult for an egg to become a bird, it is even more challenging for a bird to learn to fly while still confined within its shell. This metaphor emphasizes the necessity of change for true development and fulfillment.

Lewis argues that human beings are akin to eggs that must eventually break free from their limitations. Merely existing as a "decent egg" is insufficient; we must embrace change and growth or risk stagnation. The idea is that we all have the potential for greatness, but it requires effort and willingness to "be hatched" and evolve into our true selves.