Time. This riddle is very old, and was actually phrased much better in J. R. R. Tolkien's The Hobbit.
This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.
Time
Time
Clouds move fast across the sky due to the wind. The wind pushes the clouds along, causing them to travel quickly.
Clouds appear to move because they are being carried along by the wind. As the wind blows, it pushes the clouds across the sky, creating the illusion of movement.
It is colder on mountains because wind pushes more clouds and cold rushes up to the top of the curve of the mountain and the same on the other side into the sky.
Africa
Clouds move due to the movement of air currents in the atmosphere. Wind carries clouds along with it, shifting and changing their shape as they are carried to different locations. The speed and direction of the clouds are dictated by the strength and direction of the prevailing winds at that altitude.
Decentralizing
A constructive plate boundary, as it pushes land upwards (which formed the mountains)
Upslope fog
When magma pushes upward, it raises overlying rock layers into dome-like structures called volcanic domes or volcanic plugs. These features are formed as the magma forces its way through the Earth's crust and solidifies before reaching the surface.
Kinetic Energy