"The Pot Watcher" (or Milk Watcher)

A circular thick glass disc with a raised rim and has a notch at one side.

This glass pot watcher (or boil alert disc) is a small, heat-resistant tempered glass disc placed at the bottom of a pot to prevent liquids like milk, water, or pasta water from boiling over. It works by creating a rattling sound as water boils and disrupting bubble formation to stop messes

A pot watcher (also known as a pot minder or milk watcher) is a small ceramic, glass or stainless steel disc placed in the bottom of a pot to prevent liquids from boiling over and to audibly alert the cook when water reaches a rolling boil. As the water boils, the disc rattles, alerting cooks—especially those with visual impairments—to reduce heat.