fluids

Fluids is the umbrella term for liquids and gases — anything that flows and takes on the shape of whatever container holds it, rather than keeping a shape of its own.

• The word comes from the Latin fluidus, meaning "to flow" — a fitting name, since flowing freely is exactly what sets fluids apart from solids.
• Water, air, honey, and even blood in your body all count as fluids, even though they flow at very different speeds.
• What lets them flow is that their particles aren't locked into fixed positions the way a solid's are — they can slide and move past one another.

Grouping liquids and gases together this way is useful in science and engineering — the same equations that describe how water flows through a pipe can often describe how air flows too.