First what does water look like on the molecular level. Water is h2o 2 hydrogen and 1 oxygen. The way water is held together is when a hydrogen particle is facing towards a oxygen particle then they are connected magnetically and stay together creating the liquid state as shown on the left.
When you boil it the particles start to shake rapidly and the hydrogen particles no longer face the oxygen particles and the connection is broken. When the connection breaks the water particle starts to move upwards because of convection and that’s how we get steam or water vapor. This is what is really happening to water when it boils.