Why do boiling liquids bubble?

The bubbles evident in a pot of boiling water are pockets of steam created by adding heat to the water. Bubbles form as a result of the surface tension of the water containing that steam. Without that surface tension, individual molecules of steam would merely escape from the surface of the water directly and no bubbles would be formed. Steam expands approximately 1600 times from the volume of the water from which it is formed.