A liquid that is even modestly supplied with proteins or yolk phospholipids will form an impressive mass of bubbles, solid enough to stand up without flowing or even slumping. However, the foam may still collapse within a minute or two. Air and water have very different densities, so when the foam is left to stand on its own, the air bubbles rise while gravity pulls the liquid in their walls in the opposite direction. This means that liquid drains from the bubble walls, which also lose water to evaporation. Eventually, the foam at the surface becomes dry, around 95% air and just 5% liquid, the bubble walls become too thin, fail, and the bubbles pop.