Molecules in a gas have lots of energy and spread out even more than molecules in a liquid. Warm water has more energy than cold water, which means that molecules in warm water move faster than molecules in cold water
Heat is essentially energy, so when you add heat to water molecules, they move around more.
Eventually, once enough heat is added, water molecules will be able to escape the Inter-Molecular Forces that hold them together and keep them as liquid. The molecules will be able to move around freely, and take on the form of steam.
If you need me to explain this in more depth, let me know. -T.B.
