Diagnosis
Grease and food particles get splattered up onto the heating coil and the metal shield around it during cooking. If you don't clean it, it can smoke or burn the next time you use it.
How to Fix It
Never submerge the main unit! Here's the safe way: 1) Unplug it and let it cool completely. 2) Fill the basket with hot water and a few drops of dish soap. 3) Carefully place the basket back into the unit (this is just to create steam). Let it sit for 10-15 minutes. 4) Remove the basket. Use a soft-bristled brush (a clean toothbrush or a pastry brush works great) to gently scrub the heating element and the interior ceiling. The steam will have loosened the gunk. 5) Wipe everything down with a damp cloth.
If That Didn't Work
For stubborn, baked-on spots, make a paste of baking soda and water. Dab it onto the spots with a cloth (keep the paste away from electrical connections), let it sit for a bit, then gently scrub with your brush and wipe clean.
Heads Up
Do not use metal scouring pads or abrasive cleaners on the heating element. You can scratch it or damage the coil. And never, ever run the unit under water.