Turkey is done when it reaches 185 degrees Fahrenheit (85 degrees Celsius). If you didn't have a pop-up timer, you could use a meat thermometer to figure out the temperature of the meat. But the pop-up timer, which often comes with a turkey that you buy at the grocery store, can be more fun because it is binary -- there's no trouble reading a pop-up timer because the answer is either "Yes" or "No"!
A pop-up timer found in a turkey or chicken normally has four parts:
- The outer case (typically white or light blue)
- The little stick that pops up (typically red)
- A spring
- A blob of soft metal similar to solder
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 185 degrees Fahrenheit (85 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!