a great example would be sound. In air, the speed of sound is about 343m/s, but in iron, the speed of sound is about 5120 m/s that's about 15 times faster than air. Why is this? Well, it is because in the air the molecules are very spread out and have a long way to travel before they can bump their neighbors. But in iron, the molecules are much much closer and have a shorter distence before they hit their neighbors.
