Liquids and gases flow freely because their particles are not held together by strong forces. In liquids, the particles are close together but not fixed in place, so they can move past each other easily. In gases, the particles are even farther apart and have even more freedom to move around.

The strength of the forces between particles determines how easily a substance flows. Solids have strong forces between their particles, so they do not flow easily. Liquids have weaker forces between their particles, so they flow more easily. Gases have very weak forces between their particles, so they flow very easily.

The temperature of a substance also affects how easily it flows. As the temperature of a substance increases, the particles move faster and have more energy. This makes it easier for the particles to move past each other, so the substance flows more easily.

The viscosity of a substance is a measure of how resistant it is to flowing. Viscosity is caused by the friction between the particles of a substance. The higher the viscosity of a substance, the harder it is for it to flow.

Liquids and gases have low viscosities, so they flow easily. Solids have high viscosities, so they do not flow easily.