Eggs are excellent at foam formation. A foam is when a gas is spread throughout a liquid and whisking eggs to produce a gas-in-liquid foam.
Whisking makes the protein in the egg white unravel and denature. This allows tiny air bubbles to be incorporated into the egg white making an egg white foam.
This denaturation is reversible as if the foam is left to stand it will collapse back into liquid egg white.
When you heat the foam, the tiny air bubbles expand and the egg protein coagulates (sets) around them. This gives firmness to the foam.
Critical points: Use fresh eggs as they are slightly acidic and will produce a more stable foam. Avoid over whisking because the air bubbles in the foam will join together and the foam will leak water.