How do I change the height of a prop?

Props and characters are not bound to the ground, they can fly!  Well, they can suggest flying.  

To raise a prop or character up and down, first select the prop you wish to move. 

Make sure you are in the 3D mode.  Then, select the triangle in the bottom left corner.  

You may notice the red triangle is now on your selected prop.  Drag the red triangle up and down to put some space between your prop and the ground.