Here's a good explanation I found here: http://forum.wordreference.com/showthread.php?t=140 Over describes a vertical relationship: if a plane flies over a given city, its location on a map would be the same as the city, the only difference would be the altitude. Above describes relative heights: imagine two houses on a hillside, a mile apart. One might be above the other if it is higher. Some examples: - The top of Everest is about 8000m above sea level, even though it's several hundred miles from the sea. - My apple tree hangs over my neighbour's garden. The apples fall into his garden. - I dream of flying above the clouds. - A cloud is passing over my house right now. If it rains, I'll get wet. ---- Now I'll explain your 2 sentences: A fan is rotating above our heads: "above" indicates relative location: so, the fan may be in the middle of the room, but you are sitting near a corner of the room. **The fan is higher than your heads** If the fan falls down, it won't hit your head. :) A fan is rotating over our: "over" describes a vertical relationship. If the fan falls down, it will hit your head. I hope this helps, especially the examples that I copied.

above --- location over --- motion

In this instance there is no difference!