When you want to blend hot liquid in the food processor or blender, you often have to work in batches‚Äîeven if you can fit it all in the work bowl at once. Otherwise, as it purees, the liquid may start coming out the edges of the food processor or the built up steam can actually blow the lid off the blender.
To prevent this from happening, fill the blender or food processor up only halfway and blend in batches. It's a good idea to let the mixture cool for a few minutes before you start, and always hold a pot holder or towel over the lid when blending. For an example of the best way to process hot soup in a blender, see our recipe for Butternut-Squash Leek Soup.