A pot of water is held in the shape of a pot because the surface of the pot is solid and the water is liquid. Liquid takes the form of an empty solid.
The water begins almost entirely as liquid.
The temperature rises slowly, aggravating the H20 molecules to bounce around more quickly.
When any part of the water reaches 100 degrees Celsius, 212 degrees Fahrenheit, the hydrogen and oxygen atoms begin to split, and the substance becomes an expansive air. Tiny bubbles on the surface of the water, within the water, and between the surface of the water and the pot, appear.
The bubbles rise because they weigh less than the water, and the force of gravity doesn't pull on them as much. They are round because the air inside the bubbles is pushing out due to heat expansion, while the denser pressure of the water pushes in and holds its shape. The bubbles rise despite the water on top wanting to push down on them.
As the water continues to boil, more and more bubbles appear, rising to the surface and popping. The bubbles pop because the surface of the water on top of a bubble at the top of a boiling pot of water is much weaker than the heat expansion of the air inside, and the heat wins and pops the bubbles.
More and more bubbles appear, transferring the H20 into oxygen and hydrogen, until all the water is gone. I don't know where the hydrogen goes.