All substances are made from particles. The arrangement, movement and closeness of these particles explain many of their properties.
Solids have a fixed shape and cannot flow because their particles are only able to vibrate about a fixed position. They cannot move from place to place.
Liquids can flow and take the shape of the bottom of their container because their particles can move over each other. For example, when water is poured into a glass, the particles of water move over each other and into the corners of the glass. The particles keep on moving over each other as the water takes the shape of the glass. The slideshow explains how:
Gases can flow to fill their container and to take its shape. This is because their particles can move in all directions. For example, when a Bunsen burner is connected to a gas tap and turned on, natural gas flows through the rubber tubing. The particles of natural gas are free to move anywhere inside the tubing, and pressure forces them through the tubing and into the Bunsen burner.