Here's why:

* Gas molecules have the most kinetic energy. Kinetic energy is the energy of motion. Gas molecules are free to move around and collide with each other, resulting in high average speeds.

* Liquids have less freedom of movement. While liquid molecules can move past each other, they are more tightly packed than gas molecules, leading to slower speeds.

* Solids have the least freedom of movement. Solid molecules are fixed in a rigid structure, vibrating in place but not moving freely.

Important Note: Even within a given state of matter, the speed of individual molecules varies. Temperature directly affects the average speed of molecules. Higher temperatures mean faster-moving molecules.