Why does boiling water bubble the way it does?

When water is heated, dissolved gases escape first at lower temperatures, forming small bubbles. As the water approaches 100°C at sea level, vapor pressure equals atmospheric pressure, allowing water molecules to vaporize not just at the surface but throughout the liquid. Bubbles form at nucleation sites, which are tiny imperfections or rough spots on the pot's surface where vaporization is energetically easier. Water vapor collects at these sites, forming bubbles that grow until buoyancy overcomes surface tension, causing them to rise and release steam at the surface. The vigorous rolling boil you see is this process happening rapidly across many nucleation sites simultaneously, with convection currents also churning the water as hotter fluid rises and cooler fluid sinks.