Gas

Gas explained in an easy to understand way:

Imagine trying to fit a pile of clothes into a suitcase that’s too small, causing you to constantly rearrange them to make everything fit. Just like the clothes that spread out to fill the available space when the suitcase is opened, a gas expands to fill the shape and volume of its container. The way clothes adapt to whatever space is available is similar to how gas molecules move freely and spread out, having no fixed shape or volume, adapting to the container they're in.