Be sure to let the soup cool for 5-10 minutes before blending. Blending the mixture while very hot can actually force the lid off of the blender container due to the expansion of the steam, causing a huge mess. It also helps to start on low and progressively increase the blender speed. Blend in batches, filling the container about half full. Removing the center insert on the top of the blender (cover the opening with a folded kitchen towel) will also help prevent a mess.