Gas

Real World Example

Imagine trying to fit a large pile of laundry into a small suitcase. Just like how the clothes can be squished and reshaped to fill any available space in the suitcase, a gas can expand or compress to fill any container it's placed in, having no fixed shape or volume. In this analogy, the clothes represent the gas molecules, the suitcase is like a container, and the flexible way the clothes fill the suitcase is similar to how gas molecules spread out evenly to occupy the entire volume available to them.