Pot Minder - Boil Over Preventer Disc - Stainless Steel Pot Watcher Disc for Stopper to Safeguard Your Cooking with Boil Control Technology, for Milk, Pack of 3, OFXDD

These are also great for monitoring boiling water when I'm steaming something.

I have a similar pot minder that works very well - with it you can actually let milk boil for a long time without it ever boiling over (as long as you don't cover the pan with a lid or turn the burner on high. Since that one costs about $8 I figured I'd try these, but that was a mistake. First problem is that they're just stamped out of metal and the edges were not polished. The resulting edge is sharp enough to cut your finger. Second problem is that milk simply boils over. Burner on medium, no lid on the pan. I had expected it to just keep boiling without rising to the rim of the pan like it does with my $8 "Stainless Steel Pot Watcher – Prevent Pasta, Soups and Milk from Boiling Over", but no dice. This set is going back and I'm ordering more of the $8 discs.

Does not work. Pot boils over and this does nothing. Makes no noise to alert you.