Wilma Jean Fields,  Milwaukee, Wisc... for her question:

What makes the bubbles when water boils ?

Maybe you thought that the white spray from the tea kettle spout was the real steam. Look more closely   without getting near enough to be scalded   and you will see an inner core of the steamy spray that seems to be quite empty. That is the real steam, for vaporized water is an invisible gas. The white spray we see is really minute particles of water condensed in the cooler air.

Naturally the water tends to reach its boiling point at the bottom of the pot, nearest the flame. Drops of water become steam and expand into airy bubbles that need 1600 times more room. This gasp of course, is much lighter than liquid water. Lighter than corky it rushes up through the water to loin the air above. It comes up in little bubbles that break on the surface.