What is the key difference between a liquid and a gas?

Gas molecules vibrate and bounce off of each other from time to time. Liquid molecules also vibrate, but they're so close together that they slide around each other as opposed to bouncing off of each other.

If poured into a flask, a liquid takes on the shape of the flask, but it does not expand. Gas will take on both the shape and the volume of the flask as it expands to fill the entire vessel. Gas is also compressible, while liquid is only compressible to an extent. The compressibility of gas occurs because it is more vulnerable to changes in temperature and pressure.

Liquids also display surface tension, while gases do not. Surface tension happens since the molecules of water on the surface of a body of water are not surrounded by other molecules on all sides and cohere more strongly to the molecules around them, creating a film on top of the water.