One day I found a huge block of ice on the ground. It contained tightly packed particles. It had a definite shape and a fixed volume.
Later that day, I came back to see my block of ice melted because of the heat! Now their was a huge puddle of water. It had a fixed volume and its particles move freely. It also takes the shape of whatever container it is in.
The day got even hotter and my puddle of water began to evaporate! Now it did NOT have a definite shape or volume. Its particles moved freely now.