Milk watcher

A milk watcher, milk saver, pot watcher, pot minder, milk guard, or boil over preventer is a cooking utensil placed at the bottom of a pot to prevent the foaming boil-over of liquids by collecting small bubbles of steam into one large bubble.

A milk watcher is a disk with a raised rim notched on one side.

Some milk savers are designed so they can be used with the obverse or reverse side facing up and appear to have two notches.

Water vapor trapped under the milk saver causes the notched side to rise up, releasing the water vapor and making a rattling noise as the saver falls back into contact with the base of the pan.

The device also rattles when boiling occurs, alerting the cook who may then lower the heat setting of the stove.