Because they felt like doing it
They had to expand or die!
They needed raw materials as their territory was too small for their large population.
They had a tradition of military conquest as an...
Well it expands when it is heated because as the atoms gain more energy from the heat, they begin to vibrate more and more which ''pushes'' the surrounding atoms away, causing the spatial volume to...
Air expands due to high temperature water. And nothing happens to the molecules. As they move faster usually from increased temperature, they become farther apart.
