On the left is a pop-up timer before cooking, and on the right is the same timer after it has popped up.

The soft metal (shown in gray in the diagram) is solid at room temperature and turns to a liquid (melts) at about 175 degrees Fahrenheit (80 degrees Celsius). When the metal turns to a liquid, it frees the end of the red stick that had been trapped in the metal. The spring pops the red stick up and you know the turkey is done.

One little-known fact is that these timers are reusable. If you dip the tip in hot water, it will re-melt the metal and you can push the pop-up piece back into the metal. Then let it cool, and the pop-up piece will be back in its original position -- ready to use again!