How does a work the disc that you put at bottom of pan to stop it boiling over?

You're likely thinking of a boil-over preventer disk, also known as a splatter screen or boiling disk. These are metal disks with small holes or slots that sit on the bottom of a pan and help prevent liquids from boiling over.

Here's how they work:

* Surface Tension: The small holes and slots break up the surface tension of the liquid. When the liquid boils and bubbles form, the surface tension keeps them together, leading to a sudden surge and overflow. The holes and slots disrupt this tension, allowing the bubbles to dissipate more evenly.

* Steam Release: The holes and slots also allow steam to escape more readily, reducing the pressure that builds up inside the pan. This further prevents the liquid from boiling over.

* Increased Surface Area: The disk increases the surface area of the pan, providing more space for the liquid to evaporate. This can help to prevent the liquid from reaching its boiling point as quickly.

In essence, boil-over preventer disks work by breaking up the surface tension of the liquid, allowing steam to escape, and increasing the surface area of the pan.

However, it's important to note that these disks are not foolproof. They won't prevent boiling over if you are cooking at a very high heat or if the liquid is very full in the pan.