Matter usually exists in one of three forms: as a gas, a liquid, or a solid. The difference between these states is the extant to which the molecules or atoms of the substance are pressed close together to interact with each other. If we think of water vapor (like steam),that's water in the gas phase-- each molecule of water is floating around, comparatively far apart from all other water molecules. In the ideal form of a gas, there's no interaction between the molecules at all. When we lower the temperature (which allows the water molecules to get closer together), the molecules start interacting with other molecules, forming transient and temporary bonds. These inter-molecular interactions hold the water molecules all close together-- forming a liquid. But the bonds are temporary and unstructured, so we can pour a liquid, and the molecules on top slide right off and into our glass. If we lower the temperature even further, the inter-molecular bonds become very ordered and stable--we start forming a crystalline lattice. For water molecules, this lattice is one we're familiar with; we form the solid, ice. Watermolecules easily stack on top of each other to form an orderedlattice, and solidify.

Vodka is a mixture of water and ethanol (usually about 40% ethanol). When we lower the temperature, the differing sizes, shapes, and affinities of the water molecules and ethanol molecules for one another makes them coming together in an ordered crystalline lattice much less favorable (in chemistry terms, the freezing point is depressed). If we continue lowering the heat, vodka will eventually freeze (depending on the brand and percentage of ethanol, somewhere between -15 and -45 degrees Fahrenheit). Most home freezers will only cool to -10 or -15 degrees, so it can be difficult to freeze drinkswith a high ethanol content.