Potatoes, pasta, or anything you want to boil in water or liquid without a lid.
Simply place a wooden spoon or spatula across the top of your pan and miraculously, the liquid in your pot bubbles up all the way to the top of the pan, but *mostly never boils over.
*I added "mostly" because it works 9 out of 10 times. High heat and rapid boil may cause it not to work on occasion.