The particles or atoms change in speed and distance with each state. For a Solid, the particles are really close together and only vibrate in place. For a Liquid they are more spread apart and they vibrate and bounce off each other because there is more room- there are also less particles than a solid. For a gas the particles are very spread out and are bouncing around like bullets there are a lot fewer particles.
So for Melted Ice (water) It is a liquid, so the particles have space in between them to move a little bit, and there are not as many particles because of that extra space.