Liquid

Imagine trying to fit a pile of clothes into a suitcase; you can squish and mold them to fit the space available. Just like those clothes, a liquid conforms to the shape of its container, filling up the space available without altering its volume. The connection is clear: both the pile of clothes and a liquid have a fixed amount of 'stuff' (volume) that can be reshaped to fit into different spaces, but they don't maintain a specific shape on their own.