How do quadcopter's fly

Just as the title states this is a little video I came across and wanted to share , for those who don't know I think it's good information...
I thank you for putting that together when you understand how the why is easier.
 
Another quick way to determine the direction each motor spins is by the direction each prop is installed , ex. the props that are installed clockwise (white dot) that motor spins in a counterclockwise direction to help keep the prop on and constantly tightened down...