Diagnosis
This happens when residues from sweet, sticky ingredients like honey or peanut butter act like glue in the threads, or if the cup was screwed on too forcefully when hot.
How to Fix It
1) Fill the stuck cup with hot, soapy water and let it sit for 10-15 minutes. This can loosen the grip. 2) Pour the water out. 3) Get a good grip. Hold the cup in one hand (use a dry dish towel for traction). With your other hand, grip the blade assembly's fins firmly. Turn the blade assembly counterclockwise. If it won't budge, try tapping the side of one of the blade fins gently with the handle of a butter knife to shock the threads loose, then try twisting again.
If That Didn't Work
Try the hot water trick again, but this time add a tablespoon of white vinegar to help break down mineral deposits. You can also try running the assembled cup under very hot tap water, focusing the stream on the joint where the blade meets the cup, to expand the plastic slightly.
Heads Up
Never use pliers or channel locks directly on the plastic—you'll crush it. If you must use a tool, wrap the blade fins in several layers of duct tape first to protect them, then use adjustable pliers gently. Better yet, store your cups and blades unscrewed to prevent this.