Fluids

flu·id  (flo̅o̅′ĭd)
n.

A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.

A fluid can take shape of any container it's in. For example, if you have a star shaped container and your pour water inside, the water will take shape of a star. Liquids take shape in containers becuase the particles inside the liquid are free to move around, therefore can be shaped.

Gases don't have a shape, but they do fill up the container because they don't have a fixed volume (or shape).The particles in gases are spread far apart from each other, causing it to fill up the entire container. 

Solids, however, don't change shape or fill a container. This is because the particles are tightly packed together, making it hard for them to move around. This is why they always stay in the same shape (unless it's a solid that can be turned into a liquid or gas, or both, like ice).