How water changes to a gas?

Answer 1

Like all other liquid to gas transitions, liquid water changes to gas:

by absorbing energy from its surroundings

or

by having energy added to it.

The molecules in a mass of water will have a distribution of energies, as will the gas above the surface. When a molecule at the surface collides with another molecule of sufficient energy, it can absorb energy from the collision and potentially acquire enough energy to escape from the liquid. If energy is being added to the liquid - as heat or agitation - there are more molecules with sufficient energy to make this happen.

Energy can also be added directly to the liquid at the surface in the form radiant energy - such as the sun shining on it. A molecule can absorb this energy directly and thus gain enough energy to escape the surface of the liquid.