Have you ever tried pureeing hot soup in your blender only to have it erupt, force the lid off and spill everywhere? Culinary weblog Chow recommends starting the blender while empty and slowly adding hot liquids to avoid explosions.
With this technique you can even puree the soup without having to attach the lid. You could blend the soup as you normally would if you held the lid down with your hand, but if your soup is truly boiling hot, you could end up burning your hand.
How to Avoid Blender Explosions [CHOW Tips]