Lets start with the basics. As everyone probably knows, the molecules in a solid have little energy and forces between the molecules hold them in a rigid, solid form. As the temperature increases, the molecules gain more energy and therefore vibrate more, breaking the weaker forces holding them together, becoming a liquid. If you give the molecules even more energy they can completely break the forces holding them in a liquid and become a gas. So for a solid to form from a liquid, the molecules must be cooled below their freezing point, allowing bonds between molecules to form and a solid to be made. The formation of ice from water starts with small crystals being formed in the liquid. The water molecules in the liquid surrounding the crystals must then bond to the molecules in the crystals to freeze as well.
